Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) + (-78) + (-81) without a calculator.
All of the numbers from 75 + 72 + .... - 72 - 75 will cancel out, so we're just left with - 78 - 81 = -159
