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Odd-only subsets with two of 1, 2, 3

Question

Consider subsets of {1,2,…,10}\{1,2,\dots,10\} in which every element is odd and which contain exactly two of the numbers 1,2,31,2,3. How many such subsets are there?

Short answer

88. The only odd pair from {1,2,3}\{1,2,3\} is {1,3}\{1,3\}, so both must be included. The other odd numbers 5,7,95,7,9 may each be in or out freely, giving 23=82^3=8 subsets.

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