1)17/20 + 23/30 + 11/60 =(17 *3 + 23 * 2 + 11 * 1)/60 = 51/60 + 46/60 + 11/60 = 108/60 = 9/5 = 1 4/5
2)3/20 + 7/30 + 9/50 =(3 * 15 + 7 * 10 + 9 * 6)/300 = 45/300 + 70/300 + 54/300 = 169/300
3)7/40 + 11/70 + 13/30 =(7 *21 + 11 * 12 + 13 * 28)/840 = 147/840 + 132/840 + 364/840 = 643/840