Find the total number of unordered triplets of positive integers satisfying
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Note that if a , b , c > 3 the given sum is impossible. Similarly, a , b , c > 1 . Thus, at least one of our integers must be 2 or 3 . WLOG, say it is a . Then we have the following cases.
a = 2 . Then b 1 + c 1 = 2 1 . We know that b > 1 . If b = 2 then c 1 = 0 , a contradiction. If b = 3 then c = 6 (first solution). If b = 4 then c = 4 (second solution). If b ≥ 5 then c 1 ≥ 1 0 3 ⟹ c ≤ 3 . The only integers meeting this criteria are 3 and 2 , which we have already considered.
a = 3 . Then b 1 + c 1 = 3 2 . We now consider possible b > 2 (if b = 2 then we are in the first case). If b = 3 then c = 3 (third solution). If b ≥ 4 then c 1 ≥ 1 2 5 ⟹ c ≤ 2 . Again, we have already considered these cases.
Thus there are 3 solutions.