# One more stats question.....?

Here is the question:

A school is administering a final examination. Assume that the length of time required to complete the test is normally distributed with a mean of 60 minutes and a standard deviation of 13 minutes. How much time must be allowed by the proctors if the school wishes to assure enough time for only 85% of the applicants to complete the exam?

Not quite sure where to start or what formula to use....

### 1 Answer

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- Dr DLv 71 decade agoFavorite Answer
if 85% of the students finish the exam, then you could say the probability of finishing is 0.85.

Or the prob of finishing within the allotted timeframe is 0.85.

So find T such that P(X < T) = 0.85

The corresponding Z value would be

Z = (T - 60) / 14

From normal tables, this value is Z =~ 1.04

T = 74.6 mins.

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