Question

If seven integers are selected from the first 12 negative integers, how many pairs of these integers will have a sum of −13?

Answer 1

At least 1, possibly as many as 3.

Explanation:

There are 6 pairs of integers among those from -1 to -12 that will sum to -13. If you choose 7 integers, you may only choose one pair, or you may choose as many as three pairs.

One to three pairs will sum to -13.