To check this answer, just substitute the 15 in for y and 200 in for x and simplify. If the left side of the equation comes out to equal 30, the answer is correct.
Well, if each book only costs 0.25, then Sonia bought 9 books John bought 24 books Keisha bought 15 books If each book costs 0.75, then Sonia bought 3 books John bought 8 books Keisha bought 5 books