Estimate 42 + 15 =: 42 rounds to 40, 15 rounds to 20, 40 + 20 = 60, 42 + 15 is about 60, Estimate 45 + 17 =: 45 rounds to 50, 17 rounds to 20, 50 + 20 = 70, 45 + 17 is about 70,

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