Answer

For the number 11, the pairs would be 0 and 11, 1 and 10, 2 and 9, 3 and 8, 4 and 7, 5 and 6. There are 6 of these.

For even numbers there are n/2 + 1 ways, where n is the number.

For odd numbers there are n/2 + 1 ways, where n is the number, and any halves are dropped.

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