How to solve this algebra problem

Question

Paint costing $20 per gallon is to be mixed with 5 gallons of paint thinner costing$3 per gallon to make a paint that can be sold for$15 per gallon. How much of the new paint mixture will be produced.

Answer 1

(5)(3) + (20)(x) = 15(x + 3)

Can you solve for x and answer?