Question

The SAT and ACT college entrance exams are taken by thousands of students each year. The scores on the exam for any one year produce a histogram that looks very much like a normal curve. Thus, we can say that the scores are approximately normally distributed. In recent years, the SAT mathematics scores have averaged around 480 with standard deviation of 100. The ACT mathematics scores have averaged around 18 with a standard deviation of 6.

a. An engineering school sets 550 as the minimum SAT math score for new students. What percent of students would score less than 550 in a typical year?
b. What would the engineering school set as comparable standard on the ACT math test?
c. What is the probability that a randomly selected student will score over 700 on the SAT math test?

Answer 1

a) 75.8% of students would score less than 550 in a typical year.

b) The comparable standard would be a minimum ACT score of 22.2.

c) 0.0139 = 1.39% probability that a randomly selected student will score over 700 on the SAT math test.

Explanation:

Normal Probability Distribution

Problems of normal distributions can be solved using the z-score formula.

In a set with mean
`\mu` and standard deviation
`\sigma`, the z-score of a measure X is given by:

`Z = (X - \mu)/(\sigma)`

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the p-value, we get the probability that the value of the measure is greater than X.

Question a:

SAT, so mean of 480 and standard deviation of 100, that is,
`\mu = 480, \sigma = 100`

The proportion is the p-value of Z when X = 550. So

`Z = (X - \mu)/(\sigma)`

`Z = (550 - 480)/(100)`

`Z = 0.7`

`Z = 0.7` has a p-value of 0.758.

0.758*100% = 75.8%

75.8% of students would score less than 550 in a typical year.

b. What would the engineering school set as comparable standard on the ACT math test?

ACT, with a mean of 18 and a standard deviation of 6, so
`\mu = 18, \sigma = 6`

The comparable score is X when Z = 0.7. So

`Z = (X - \mu)/(\sigma)`

`0.7 = (X - 18)/(6)`

`X - 18 = 0.7*6`

`X = 22.2`

The comparable standard would be a minimum ACT score of 22.2.

c. What is the probability that a randomly selected student will score over 700 on the SAT math test?

This is 1 subtracted by the p-value of Z when X = 700, so:

`Z = (X - \mu)/(\sigma)`

`Z = (700 - 480)/(100)`

`Z = 2.2`

`Z = 2.2` has a p-value of 0.9861.

1 - 0.9861 = 0.0139

0.0139 = 1.39% probability that a randomly selected student will score over 700 on the SAT math test.