Statistics question, please help asap?

Question

The time to complete a standardized exam is approximately normal distributed with a mean of 100 minutes and a standard deviation of 15 minutes. The time, to the nearest minute, that should be given to complete the exam so that 90% of the students will complete the exam on time is?

Answer 1

Look into the Standard Normal Distribution table to determine the z score for an area (probability) of 0.9 (you may have to interpolate), or use a STAT menu on a graphing calculator to determine the needed z score. (INVNORM command on TI-84, if I remember correctly)

 

Then solve z = (x-μ) / σ for x, because you have all other quantities as  numbers.