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Blackboard Sums II

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

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Tags: Numbers
If you enjoyed this puzzle, check out Sunday Afternoon Maths LXI,
puzzles about division, or a random puzzle.
© Matthew Scroggs 2017