Today Puzzle #660
Puzzle No. 660 鈥� Thursday 23 January
Six of the seven numbers 11, 20, 15, 25, 16, 19 and 17 are divided into three groups of two numbers so that the sum of the two numbers in each group is the same. Which number is not used?
Today’s #PuzzleforToday has been set by the Senior Kangaroo mathematics competition, run in the UK by the UK Mathematics Trust.
Click here for the answer
15. Let the number not used be x. The sum of the seven numbers is 123 which is divisible by 3. The six numbers used are divided into three pairs with the same sum so 123-x is also divisible by 3. This manes that x is divisible by 3 and the only number in the list that is divisible by 3 is 15. The remaining six numbers can then be paired as 11 and 25, 20 and 16, 19 and 17 all with sum 36.
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