Scores on a test have a mean of 66 and Q3 of 81. The scores have a distribution that is approximately normal. Find the standard deviation. Round your answer to the nearest tenth.

Answer 1

Q3 = 75 percentile

In a table in the back of your stat book labeled something like "areas under normal distribution," find the Z score equivalent. Put value in formula below.

Z = (x - μ)/ SD = (81-66)/SD

Solve for SD.

Answer 2

(81-66)/SD = 0.67(z score of 0.75)
15/SD= 0.67
15/0.67
SD=22.3880