Question of the day
2026-08-12
Brain Teasers
Quick sum: what is 1 + 2 + 3 + … + 100?
Answer it out loud first — like you would in the room. Then check yourself:
Reveal the model answer
Model answer
5,050.
Gauss pairing: pair the ends — 1+100 = 101, 2+99 = 101, 3+98 = 101 … There are 50 such pairs, so 50 × 101 = 5,050.
General formula: sum of 1 to n = n(n + 1)/2. Check: 100 × 101 / 2 = 5,050. ✓
Useful variant: sum of 1 to 20 = 210; sum of 1 to 10 = 55. This formula also powers the 2-egg drop and handshake problems.
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