Boreal is WRONG! The answer is NOT $15,937.50!
Ignore her/his response.
Let x = "How much would he have to sell in a month". Then your equation is (x-1550)*0.08 + 625 = 1900 dollars. Simplify and solve for x: 0.08X - 0.08*1550 + 625 = 1900 0.08x = 1900 - 625 + 0.08*1550 x == (one click MS Excel) = 17487.50. Answer. He has to cell on $17487.50 in a month. Check. (17487.50-1550)*0.08 + 625 = 1900 dollars. ! Correct !