Algebra word problem

Question

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 bottle for $1 and black water bottle for $2 and earned sum of $35. Find the number of black waters bottles he has

Answer 1

Let x = number of white and y = number of black

x + y = 20

1x + 2y = 35

Since x + y = 20, y = 20 - x.

So, x + 2(20 - x) = 35

Solve for x.