statistics
posted by abby on .
The time to complete a standardized exam is approximately normal with a mean of 70 minutes and a standard deviation of 10 minutes. Using the 689599.7 rule, if students are given 90 minutes to complete the exam, what percentage of students will not finish?

Here is what they mean by that rule:
68% finish within one std. deviation.
95% finish within two std. deviations (50 to 90 minutes in this case).
99.7% finish withing three std. deviations.
Considering the second group, half (47.5%) will finish within 70 to 90 minutes. Since the fraction above the mean is 50% for a normal distribution, that means 2.5% will not finish at all in 90 minutes.
A more exact value, using a calculator, is 2.3%