Explain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + ... + (-69) + (-72) + (-75) without a calculator.
+53 The 75 cancels with the -75. The 72 cancels with the -72. Since the difference between the two numbers is 3, and 3|75, all the numbers cancel out and sum to 0.
