1) 5 * 48 = 5 * (40 + 8) = 5 * 40 + 5 * 8 = 200 + 40 = 240
2) 240 : 20 = 24 : 2 = 12
3) 12 * 6 = (10 + 2) * 6 = 10 * 6 + 2 * 6 = 60 + 12 = 72
4) 72 : 18 = 4
5) 4 * 15 = 4 * (10 + 5) = 4 * 10 + 4 * 5 = 40 + 20 = 60
6) 60 : 12 = 5