Glenn and Jason each have a collection of tennis balls. Glenn said that if Jason would give him 4 of his balls they would have an equal number; but, if Glenn would give Jason 4 of his balls, Jason would have 3 times as many balls as Glenn. How many balls does Jason have?

Let Glenn have G balls and Jason have J balls. If Jason gives 4 to Glenn: G+4 = J-4, so G = J-8. If Glenn gives 4 to Jason: J+4 = 3(G-4). Substituting: J+4 = 3(J-8-4) = 3J-36. Solving: 2J = 40, J = 20. Checking with J=20: Glenn has 12. After Jason gives 4: both have 16. After Glenn gives 4: Jason has 24, Glenn has 8. 24 = 3×8. Correct.