To find the average, you have to add everything up and divide by the amount that you have. In this case, you need to add together 160.5, 165.75 and 152.25, and then divide by 3.
160.5+165.75+152.25= 478.5
You've then got to divide this by three.
478.5/3= 159.5
Therefore, the average hour worked each month bu the employee is 159.5
Hope this helps :)