9: John has two colored varieties of water bottles: white water bottles and black water bottles. The total number of water bottles is 20. He sold a white water bottles for $1 and a black water bottles for $2 and earned a sum of $35. Find number of black water bottles he has.
Due to the fact that he only had 20 bottles. With him selling the black at 2 dollars. 15 would end up being 30 dollars and the whites making up the rest of the money.