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18
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Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) + (-78) + (-81) without a calculator.

 Nov 17, 2022
 #1
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All of the numbers from 75 + 72 + .... - 72 - 75 will cancel out, so we're just left with - 78 - 81 = -159

 Nov 18, 2022

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