Step 1) Make the equation describing the situation:
y = 0.045 x + 250
Where x is the sales and y is his total earning.
if,
y = 500 (total earning)
then,
500 = 0.045 x + 250
500 - 250 = 0.045 x
250 = 0.045 x
x = 250/0.045
x = $5,555.55
The total value of his sales should be $5,555 to earn a total of $500.