Pippa thinks of a number. She adds 1 to it to get a second number. She then adds 2 to the second number to get a third number, adds 3 to the third to get a fourth, and finally adds 4 to the fourth to get a fifth number. Pippa’s brother Ben also thinks of a number but he subtracts 1 to get a second. He then subtracts 2 from the second to get a third, and so on until he has five numbers. They discover that the sum of Pippa’s five numbers is the same as the sum of Ben’s five numbers. What is the difference between the two numbers of which they first thought?
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Junior Mathematical Olympiad 2013 (2013)
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