51 : 17 = 3;
68 : 4 = (40 + 28) : 4 = 40 : 4 + 28 : 4 = 10 + 7 = 17;
96 : 16 = 6.
64 : 4 = (40 + 24) : 4 = 40 : 4 + 24 : 4 = 10 + 6 = 16;
13 * 6 = (10 + 3) * 6 = 10 * 6 + 3 * 6 = 60 + 18 = 78;
80 : 5 = (50 + 30) : 5 = 50 : 5 + 30 : 5 = 10 + 6 = 16.