Let x = commissions
Let y = total earnings
y = 0.20x + $750. Plug your numbers in.
$4,000 = 0.20x + $750. Subtract 750 from each side.
$3250 = 0.20x. Divide each side by 0.20.
$16,250 = x.
The salesperson needs to sell $16,250 in commissions to earn $4,000 in total earnings each month