For this problem we know that the salary per week is 500 and he gets a commission of 4% for each sale over 800. During one week the total sales are 1330 and we want to estimate the gross earnings for the week.
Taking in count the info given we can set up the following equation for the total amount recieved:
Where A represent the amount recieved and S the sales. For this case S=1330 and replacing we got: