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Proposition. (The Pigeonhole Principle, simple version.)
If pigeons stay in holes then there is a hole with at least two pigeons.
We shall use the pigeonhole principle:
Consider the 5 pairs of numbers
In each case, the sum of the two numbers is
These pairs will serve as our "pigeon-holes". If we select 6 distinct integers (i.e., the "pigeons") from the integers 3 to 12, inclusive -- then by the pigeonhole principle, at least two of them must be in the same pair. Since the 6 integers chosen were distinct, we have found two that add up to 15.