How many unique sums can be formed by adding any three different numbers from the set {4,6,8,9,12,15,18}?
(18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 41, 42, 45)==24 unique sums
