The consecutive integers from 1 to 29 are written on a board. After one number is erased, the average of the remaining 28 numbers is exactly 15. Which number was erased?
15
The sum of integers 1 through 29 is (29 × 30) / 2 = 435. The sum of the 28 remaining numbers with an average of 15 is 28 × 15 = 420. The erased number is 435 - 420 = 15.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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