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Problem 3 - Entrance Test

If x^2 + y^2 = 16 and x - y = 4, then what is the value of xy?

Correct: C

Square the second equation: (x - y)^2 = x^2 - 2xy + y^2 = 16. We know x^2 + y^2 = 16, so substitute that into the equation: 16 - 2xy = 16. Thus, -2xy = 0, so xy = 0.