Compute the sum of the first hundred numbers, 1+2+3+...+98+99+100, without a calculater, in under three minutes. In other words, find a shortcut.
Answer is:
Add the numbers up in a different order: 1+100 + 2+99 + 3+98 + ... + 50+51. This produces 50 copies of 101, hence 50×101, or 5,050.
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