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The integers 1, 2, …., 40 are written on...

The integers 1, 2, …., 40 are written on a blackboard. The following operation is then repeated 39 times, In each repetition, any two numbers, say a and b, currently on the blackboard are erased and a new number `a+b-1` is written. What will be the number left on the board at the end?

A

820

B

821

C

781

D

819

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The correct Answer is:
D
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