John has two jobs. For daytime work at a jewelry store he is paid $200 per month, plus a commission. His monthly commission is normally distributed with mean $600 and standard deviation $40. At night he works as a waiter, for which his monthly income is normally distributed with means $100 and standard deviation$30. John's income levels from these two sources are indepdent of each other. For a given month, the probability is 0.9 that john's income as a waiter is less how much in a given month?