Answer:
4 gallons of black paint
Explanation:
Ok so here I'd use proportions:
If he used 3 parts white and 2 parts black we get this fraction:
3/2
he buys 6 parts white but we don't know how much black paint we need so we can set this equal to each other like so:
the 6 parts white is set equal to 3 parts white now we cross multiply to find x
3x = 12
divide
x = 4
alternatively you could also just divide 6 by 3, you get 2 then multiply 2 by 2(black paint) and you would get the same answer