Studies have shown that an ordinary share’s annual rate of return is approximately normally distributed. Assume you have invested in the shares of a company for which the annual return averages 15% with a standard deviation of 10%.
a) What is the probability that your annual return will exceed 30%?
b) What are the chances that your return will be negative?
c) You decide you will re-invest for a further 12 months if the return in the first year is at least 2½ %. What is the probability that you will re-invest?
d) Determine the return that has a 90% chance of being exceeded.

I have been able to do until part c, but I don’t understand how part 4 is calculated.


PLEASE HELP ASAP
Thank you