The quantity y varies directly as x and inversely as z. When x is 10 and z is 4, y is 15. What is y when x is 20 and z is 6?

Direct variation with x and inverse variation with z means y = kx/z for some constant k. Using x = 10, z = 4, and y = 15 gives k = 15 × 4 ÷ 10 = 6. Then y = 6 × 20 ÷ 6 = 20, so the correct choice is 20.

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