Challenge Problem of the Month

Answer correctly to win!

I will choose one person out of all the people who answer correctly to win a free Manhattan GRE Strategy Guide. If you're keen on calculations or on winning free things, you should try your hand this month.

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This Month's Problem: "Not Positive"

The operation ◊ is defined for all real numbers a and b as ab = ba. Which of the following CANNOT be positive?




(a ◊ 1)(1 ◊ b)

(a + 1) ◊ (b + 1)

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