Posted by **Abby** on Monday, March 29, 2010 at 11:19am.

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.

- statistics -
**PsyDAG**, Monday, March 29, 2010 at 12:05pm
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.

- statistics -
**caroline**, Tuesday, March 29, 2016 at 10:58pm
(81-66)/SD = 0.67(z score of 0.75)

15/SD= 0.67

15/0.67

SD=22.3880

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