Normal Distribution (Edexcel A Level Maths: Statistics): Exam Questions

Using the data from the large data set, Simon produced the following summary statistics for the daily mean air temperature, °C, for Beijing in 2015

Show that, to 3 significant figures, the standard deviation is 5.19 °C.

Simon decides to model the air temperatures with the random variable

Using Simon’s model, calculate the 10th to 90th interpercentile range.

Simon wants to model another variable from the large data set for Beijing using a normal distribution.

State two variables from the large data set for Beijing that are not suitable to be modelled by a normal distribution. Give a reason for each answer.

The following diagram shows the distribution of heights, in cm, of adult women in the UK:

The distribution of heights follows a normal distribution, with a mean of 162 cm and a standard deviation of 6.3 cm.

 Write down the values of the height that correspond to:

(i) the maximum point on the curve 

(ii) the points of inflection on the curve

Use the properties of the normal distribution to suggest a range of heights within which the heights of

(i) 68%

(ii) 95%

(iii) nearly all

of adult women in the UK will fall.

For the random variable  find the following probabilities:

(i)

(ii)

(iii)

For the random variable  find the following probabilities:

(i)

(ii)

(iii)

The weight, g, of a chocolate bar produced by a certain manufacturer is modelled as .

Find:

(i)

(ii)

Heledd buys a pack containing 12 of the chocolate bars.  It may be assumed that the 12 bars in the pack represent a random sample.

Find the probability that all of the bars in the pack have a weight of at least 195 g.

The random variable . 

 Find the value of , to 2 decimal places, such that:

(i)

(ii)

(iii)

The random variable .

Find the value of  and the value of  , each to 2 decimal places, such that:

(i)

(ii)

Use a sketch of the distribution of  to explain why .

The test scores, , of a group of RAF recruits in an aptitude test are modelled as a normal distribution with

(i) Find the values of  and  such that  and 

(ii) Hence find the interquartile range of the scores.

Those who score in the top 30% on the test move on to the next stage of training.

One of the recruits, Amelia, achieves a score of 231. Determine whether Amelia will move on to the next stage of training.

For the standard normal distribution , find:

(i)

(ii)

(iii)

The random variable .

By using the coding relationship between  and , re-express the probabilities from parts (a) (i), (ii) and (iii) in the forms ,  and respectively, where , ,  and  are constants to be found.

The table below shows the percentage points of the normal distribution.  The values  in the table are those which a random variable  exceeds with probability .

(i) Use the percentage points table for the standard normal distribution to find the value of  for which 

(ii) Use your answer to part (a)(i) along with the properties of the normal distribution to work out the values of and  for which    and  .

The weights,  kg, of coconuts grown on the Coconutty As They Come coconut plantation are modelled as a normal distribution with mean 1.25 kg and standard deviation 0.38 kg.  The plantation only considers coconuts to be exportable if their weight falls into the 20% to 80% interpercentile range.

Use your answer to part (a)(ii) to find the range of possible weights, to the nearest 0.01 kg, for an exportable coconut.

A machine is used to fill cans of a particular brand of soft drink.  The volume,  ml, of soft drink in the cans is normally distributed with mean 330 ml and standard deviation  ml.  Given that 15% of the cans contain more than 333.4 ml of soft drink, find:

the value of 

Six cans of the soft drink are chosen at random.

Find the probability that all of the cans contain less than 329 ml of soft drink.

The random variable . It is known that and 

Find the values of  and  for which  and where  is the standard normal variable.  Give your answers correct to 4 decimal places.

Use your answers to part (a), along with the relationship between and , to show that the following simultaneous equations must be true:

By solving the simultaneous equations in (b), determine the values of and . Give your answers correct to 2 decimal places.

A manufacturer uses a machine to make metal rods.

The length of a metal rod, cm, is normally distributed with

• a mean of 8cm

• a standard deviation of cm

Given that the proportion of metal rods less than 7.902 cm in length is 2.5%, show that to 2 decimal places.

Calculate the proportion of metal rods that are between 7.94 cm and 8.09 cm in length.

The cost of producing a single metal rod is 20p

A metal rod

• where is sold for scrap for 5p

• where is sold for 50p

• where is shortened for an extra cost of 10p and then sold for 50p

Calculate the expected profit per 500 of the metal rods.

Give your answer to the nearest pound.

The heights of females from a country are normally distributed with

• a mean of 166.5cm

• a standard deviation of 6.1 cm

Given that 1% of females from this country are shorter than cm, find the value of .

Find the proportion of females from this country with heights between 150cm and 175cm.

A female, from this country, is chosen at random from those with heights between 150 cm and 175cm.

Find the probability that her height is more than 160cm.

The following diagram shows the distribution of heights, in cm, of adult men in the UK:

The distribution of heights follows a normal distribution, with a mean of 175.3 cm and a standard deviation of 7.6 cm.

Write down the values of the height that correspond to:

(i) the line of symmetry of the curve.

(ii) the points of inflection on the curve.

Use the properties of the normal distribution to determine whether each of the following statements is likely to be true. In each case give a reason for your answer.

(i) 95% of adult men in the UK will have a height less than 190.5 cm.

(ii) 68% of adult men in the UK will have a height between 167.7 cm and 182.9 cm.

(iii) 81.5% of adult men in the UK will have a height between 160.1 cm and 182.9 cm.

Paul, a renowned mathematics educationalist, has a height of 196 cm.

Find the percentage of adult men in the UK who have a height that is less than Paul’s.

The weight, kg, of the feed in a sack of partridge feed produced by a certain manufacturer is modelled as .

Find:

(i)

(ii)

Roger buys ten sacks of the manufacturer’s partridge feed to feed to the partridges who have begun showing up at his backyard bird feeding station.

Using one of your answers to part (i), along with the properties of the normal distribution, find the probability that all ten sacks contain feed with a weight that is within 250 g of 20 kg.

The random variable . 

Find the value of , to 3 decimal places, such that:

(i)

(ii)

The random variable .

Find the value of  and the value of , each to 3 decimal places, such that:

(i)

(ii)

Use a sketch of the distribution of , along with the properties of the normal distribution, to explain why .

The test scores, , of a group of Royal Navy recruits in an aptitude test are modelled as a normal distribution with  

Find the interquartile range of the scores.

Those who score in the top 1% on the test are eligible to join the submarine service.

One of the recruits, Mervyn, is a keen would-be submariner. He achieves a score of 750 on the test.  Determine whether Mervyn will be eligible to join the submarine service.

For the standard normal variable    the function  is defined by

Find:

(i)

(ii)

(iii)

The random variable .

Use your answers from part (a), along with the relationship between  and , to work out the following probabilities:

(i)

(ii)

(iii)

The table below shows the percentage points of the normal distribution. The values in the table are those which a random variable  exceeds with probability .

Use the percentage points table for the standard normal distribution, along with the general properties of the normal distribution, to work out the values of  and  for which and  .

A new agricultural standardisation scheme has been proposed that will measure the bendiness of bananas in terms of a unit called the ‘bent’.  It is found that the bendiness measurements, , of bananas grown on the Yes We Have Some Bananas banana plantation can be modelled as a normal distribution with mean 55.3 bents and standard deviation 6.1 bents.  Because the owners of the plantation are committed to making sure that the bananas they export are neither too bendy nor not bendy enough, the plantation only considers bananas to be exportable if their bendiness falls into the 15% to 85% interpercentile range of the plantation’s banana bendiness measurements.

Use your answer to part (a) to find the range of possible bendiness measurements, to the nearest hundredth of a bent, for an exportable banana.

A machine is used to fill bags of potatoes for a supermarket chain.  The weight, kg, of potatoes in the bags is normally distributed with mean 3 kg and standard deviation kg. 

Given that 7% of the bags contain a weight of potatoes that is at least 50 g more than  the mean, find:

Twelve of the bags of potatoes are chosen at random.

Find the probability that not more than one of the bags will contain less than 2.96 kg of potatoes.

The random variable  It is known that  and 

Use the relationship between  and the standard normal variable  to show that the following simultaneous equations must be true:

By solving the simultaneous equations in (a), determine the values of  and .

The lifetime, hours, of a battery has a normal distribution with mean 18 hours and standard deviation 4 hours.

Alice’s calculator requires 4 batteries and will stop working when any one battery reaches the end of its lifetime.

Find the probability that a randomly selected battery will last for longer than 16 hours.

At the start of her exams Alice put 4 new batteries in her calculator.

She has used her calculator for 16 hours, but has another 4 hours of exams to sit.

Find the probability that her calculator will not stop working for Alice’s remaining exams.

Alice only has 2 new batteries so, after the first 16 hours of her exams, although her calculator is still working, she randomly selects 2 of the batteries from her calculator and replaces these with the 2 new batteries.

Show that the probability that her calculator will not stop working for the remainder of her exams is 0.199 to 3 significant figures.

A health centre claims that the time a doctor spends with a patient can be modelled by a normal distribution with a mean of 10 minutes and a standard deviation of 4 minutes.

Using this model, find the probability that the time spent with a randomly selected patient is more than 15 minutes.

The health centre also claims that the time a dentist spends with a patient during a routine appointment, minutes, can be modelled by the normal distribution where

Using this model,

(i) find the probability that a routine appointment with the dentist takes less than 2 minutes

(ii) find

(iii) hence explain why this normal distribution may not be a good model for .

The dentist believes that she cannot complete a routine appointment in less than 2 minutes.

She suggests that the health centre should use a refined model only including values of .

Find the median time for a routine appointment using this new model, giving your answer correct to one decimal place.

A machine puts liquid into bottles of perfume. The amount of liquid put into each bottle, ml, follows a normal distribution with mean 25 ml.

Given that 15% of bottles contain less than 24.63 ml, find, to 2 decimal places, the value of such that

The following histogram shows the distribution of weights, in grams, of a population of dormice in the UK:

(i) Explain why it would be appropriate to use a normal distribution to model the distribution of weights of the dormouse population.

(ii) Explain how the histogram could be altered so as to better approximate the smooth curve of the normal distribution.

The mean and standard deviation for the weights of the dormouse population are calculated to be 20.5 g and 2.6 g respectively.  A normal curve is drawn corresponding to these values.

Write down the values of the weight that correspond to the line of symmetry and the points of inflection of the normal curve.

Use the properties of the normal distribution to determine whether each of the following statements is likely to be true. In each case give a reason for your answer.

(i) 84% of the dormice have a weight that is less than 23.1 g.

(ii) More than 99% of the dormice have a weight that is between 15.3 g and  28.3 g.

(iii) 18.5% of the dormice have a weight that is either less than 17.9 g or greater than 25.7 g.

The weight,  kg, of the feed in a sack of pheasant feed produced by a certain manufacturer is modelled as   .

Roger buys twelve sacks of the manufacturer’s pheasant feed to feed to the pheasants who have begun showing up at his backyard bird feeding station.

Find the probability that all twelve sacks contain feed with a weight that is within 35 g of 20 kg.

The random variable . 

Find the value of , to 3 decimal places, such that:

(i)

(ii)

 The random variable .

 Find the value of  and the value of , each to 3 decimal places, such that:

(i)

(ii)

Use a sketch of the distribution of , along with the properties of the normal distribution, to explain why .

The distribution of the test scores, , of a group of British Army officer cadets on an aviation aptitude test is modelled as a normal distribution with  .

Only cadets who score in the top 10% on the test are eligible to proceed directly to helicopter pilot training.  Cadets whose scores are between the 40th and 90th percentiles, however, are eligible to resit the test in an attempt to improve their scores.

Given that it is only possible to receive an integer number of marks as a score on the test, determine the range of test scores for which cadets would be eligible to resit the test.

(i) Find 

(ii) The maximum score it is possible to receive on the test is 200.  Use this fact, and your answer to part (b)(i), to criticise the model being used for the score distribution.

For the standard normal variable    the function  is defined by

The constants  and  are positive real numbers.

Find an expression for each of the following probabilities, giving your answers as simply as possible in terms of   and

(i)

(ii)

The random variable .

Find an expression for each of the following probabilities. You should give your answers as simply as possible in terms of the function where the argument of the function should in each case be given in terms of  and . 

(i)

(ii)

(iii)

The table below shows the percentage points of the normal distribution.  The values  in the table are those which a random variable  exceeds with probability .

The Monkey Puzzle Tree Marketing Board has proposed a new scheme that will measure the puzzlingness of monkey puzzle trees in terms of a unit called the ‘fuddle’.  It is found that the puzzlingness measurements, , of monkey puzzle trees grown on the We ♥ Puzzling Monkeys monkey puzzle tree plantation can be modelled as a normal distribution with mean  fuddles and standard deviation  fuddles.  Because the owners of the plantation are committed to making sure that the monkey puzzle trees they sell to gardeners are neither too puzzling nor not puzzling enough, the plantation only considers monkey puzzle trees to be saleable if their puzzlingness falls into the 10% to 97.5% interpercentile range of the plantation’s monkey puzzle tree puzzlingness measurements.

Using the percentage points table for the standard normal distribution, or otherwise, find the range of possible puzzlingness measurements for a saleable monkey puzzle tree.  Your answer should be given in terms of  and .

A machine is used to produce the 3-metre barge poles sold by the You Would Touch It With One Of Ours barge pole company.  The actual length,  m, of the barge poles is normally distributed with mean 3 m, standard deviation  m, and interquartile range 0.01712 m.

Twenty of the barge poles are chosen at random.

Find the probability that at most two of the barge poles will be shorter than 3 m by 1 cm or more.

An archaeologist has devoted his life to studying ancient Greek vases produced by a particular Boeotian pottery workshop.  The vases were made to a standard pattern, and after measuring a very large number of them the archaeologist has found that 5% of the vases have a mass greater than 2.237 kg, while only 1% of them have a mass less than 1.906 kg.

Given that the masses of the vases may be assumed to be distributed normally, find the mean and standard deviation of the distribution.

The archaeologist has found that vases made by the workshop with a mass less than 1.93 kg are particularly fragile and require special care.

 A museum has just purchased a collection of  vases produced by the workshop.  The  vases may be assumed to be a random sample.

Given that there is a less than 15% chance that the collection contains vases that are particularly fragile and require special care, find the greatest possible value of . Your answer should be supported by clear algebraic working.