1 1/2+4 1/4+3 1/8= 3/2 + 17/4 + 25/8 = 12/8 + 34/8 + 25/8 = 71/8 = 8 7/8 = 8,875
6 5/9+2 7/18-7 11/36= 59/9 + 43/18 - 263/36 = 236/36 + 86/36 - 263/36 = 59/36 = 1 23/36
21 13/15-11 7/12+18 11/20= 328/15 - 139/12 + 371/20 = 1312/60 - 695/60 + 1113/60 = 1730/60 = 28 50/60