The incomes of trainees at a local mill are normally distributed with a mean of $1,100 and a standard deviation $150. what percentage of trainees earn less than $900 a month?

In this problem we are given the mean of $1100, SD of $150 and x equal to $900. In this case, we need to use the z-score table to answer the problem:

z = (x-mean) / sd

z = (900-1100) / 150

z = - 1.33

from z-table, the probability at the left of z = - 1.33 is equal to 9.18%