Statistics. Normal distribution.

A school student investigated how long he actually had to spend on homework assignments, which were normally for half hour periods. He found that the times were approximately normally distributed, with mean 35 minutes and standard deviation 8 minutes. Using this model and assuming independence between assignments, find (a) the probability that one particular assignment will take less than 25 minutes, (b) the time in which 90% of all assignments can be completed, [ Ans: (a) 0.1056 (b) 45.3 minutes ]