a and c are correct

b) If X and Y are independent random variables that are normally distributed (and therefore also jointly so), then their sum is also normally distributed. link

X has mean 90,000 and sd = 20,000

Y has mean 25,000 and sd = 5,000

Z = (X + Y) has mean 90,000 + 20,000 and

Once you have the mean and sd you should be able to do part d