10 10/13 - 6 3/10 = 140/13 - 63/10 =(140 × 10 - 63 × 13)/130 = 1400/130 - 819/130 = 581/130 = 4 61/130
21 7/18 - 1 1/24 = 385/18 - 25/24 = (385 × 4 - 25 × 3)/72 = 1540/72 - 75/72 = 1465/72 = 20 25/72
13 - 9 13/24 = 13/1 - 229/24 =(13 × 24 - 229 × 1)/24 = 312/24 - 229/24 = 83/24 = 3 11/24