The average length of time required for students to complete a test in an Introd

Question

The average length of time required for students to complete a test in an Introductory Statistics course is found to be 65 minutes with a standard deviation of 7 minutes. The time required to complete the test follows a Normal distribution. Assume that students are allowed 80 minutes to complete the test. The instructor would like to marginally increase the difficulty level of the test by increasing the standard deviation so that 95% of the students have sufficient time to complete the test. What does the increased standard deviation have to be?

I put 9 as my answer before but I do not understand why my teacher said it was wrong.

Explanation / Answer

By empirical rule

95% of the distribution lies in between

mean-2*sd and mean+2*sd

65-2*7 and 65+2*7

51 and 79

it is 2*standard devaition

2*7=14

increasing the standard deviation to 14 so that 95% of the students have sufficient time to complete the test.

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