statistics
posted by abby .
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%
Respond to this Question
Similar Questions

statistics
time required to complete a statistics exam is uniformly distributed between 60 and 150 minutes. What is the probability that a student will take longer than 120 minutes to complete the exam 
statistics
The times taken to complete a statistics test by all students are normally distributed with a mean of 120 minutes and a standard deviation of 12 minutes. Find the probability that the mean time taken to complete this test by a random … 
statistics
The times taken to complete a statistics test by all students are normally distributed with a mean of 120 minutes and a standard deviation of 12 minutes. Find the probability that the mean time taken to complete this test by a random … 
DBU
The mean time to complete a psychology exam is 34 minutes with a standard deviation of 8 minutes. Assume the distribution is normal. Find the probability of a student taking less than 28 minutes. 
statistics
historically, the mean timr to complete this statistics test is 54 minutes with a standard deviation of 9 minutes. how much time should i allow, if my goal is for 95% of the students to be able to complete this test 
Math
The mean time to complete a certain stat quiz is 25 minutes, with a standard deviation of 5 minutes. Find the probability of a student taking. 1.) less than 15 minutes. 2.) Between 15 and 25 minutes. 3.)more than 25 minutes. 
college statistics
the average length of time required for students to complete a test in a an introductory statistics course is found to be 50 minutes with a standard deviation of 12 minutes. if students are allowed 80 minutes to complete the test , … 
Math
The time needed to complete a final examination in a particular college course is normally distributed with a mean of 83 minutes and a standard deviation of 13 minutes. Answer the following questions. What is the probability of completing … 
statistics
The time needed to complete a final examination in a particular college course is normally distributed with a mean of 79 minutes and a standard deviation of 8 minutes. Answer the following questions. Assume that the class has 60 students … 
Math
According to these data, the mean time students spent on the exam was 94.1 minutes, and the standard deviation was 24.23 minutes. Suppose the exam proctor realized after compiling these data that he had used the wrong start time in …