cate din submultimile multimii { 1 , 2 , ...15 } au exact trei elemente a caror suma e divizibila cu 3 ?

Răspuns :

{1,2,3}, {1,2,6}, {1,2,9}, {1,2,12}, {1,2,15}, {2,3,4}, {2,3,7}, {2,3,10}, {2,3,13}, {3,4,5}, {3,4,8}, {3,4,11}, {3,4,14}, {4,5,6}, {4,5,9}, {4,5,12}, {4,5,15}, {5,6,7}, {5,610}, {5,6,13}, {6,7,8}, {6,7,11}, {6,7,14}, {7,8,9}, {7,8,12}, {7,8,15}, {8,9,10}, {8,9,13}, {9,10,11}, {9,10,14}, {10,11,12}, {10,11,15}, {11,12,13}, {12, 13,14}, {13,14,15}=35 submultimi
Am atasat rezolvarea.
Vezi imaginea LUCASELA