The incomes of trainees at a local mill are normally idstributed with a mean of $1100. And a standard deviation of $150. . What percentage of trainees earn less than $900 a month.

Just use z=\frac{x-{\mu}}{\sigma} and look it up in the z-table.

x=900, \;\ {\mu}=1100, \;\ {\sigma}=150