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Saving for my birthday

Saving for my birthday

On January 1, I started saving for my birthday party. On the first day I saved a coin, on the second day I saved two coins, on the third day three coins and so on until the day of my birthday when I managed to save 276 coins.

What day is my birthday?

Solution

January 23

276 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20 + 21 + 22 + 23.