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Permanent link to this puzzle: mscroggs.co.uk/puzzles/141

Source: Inspired by Problem Solving 101 by Stephen Muirhead (Chalkdust Magazine)

The numbers 1 to 20 are written on a blackboard. Each turn, you may erase two adjacent numbers, \(a\) and \(b\) (\(a\) is to the left of \(b\)) and write the difference \(a-b\)
in their place. You continue until only one number remains.

What is the largest number you can make?

If you enjoyed this puzzle, check out Sunday Afternoon Maths LXI,

puzzles about division, or a random puzzle.

puzzles about division, or a random puzzle.