Your firm currently sells 130 units a month at a price of $210 a unit. You think you can permanently increase your sales by an additional 50 units per month if you switch to a net 30 credit policy. The monthly interest rate is .6 percent and your variable cost per unit is $125. What is the net present value of the proposed credit policy switch?

The answer is 674,783. Whoever shows me how to get this gets the points.

Please show steps and the equations you used.