Four kids can wash three whiteboards in 20 minutes. How long would it take, in minutes, for one kid to wash six whiteboards?
Think hard. Detailed solution:
160
One kid can wash three whiteboards in 20 * 4 = 80 minutes.

One kid can wash six whiteboards in 80 * (6 / 3) = 160 minutes.

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Thus, the answer is 160