C. "David's equation is correct, because their spending will be multiplied by the number of months and then subtracted from their savings"
they are technically saying "Look we have 12,350 to use. How long (y) will the savings (12,350) last after using 240 each month (x)
so the amount they use *PER* month gets subtracted from the savings because they are using that money to pay with
Hope this helps :)