20/21 + 3/7 = 20/21 + 9/21 = 29/21
7/9 - 4/15 = 35/45 - 12/45 = 23/45
4 - 7/19 = 76/19 - 7/19 = 69/19
10 - 5 12/35 = 10 - 187/35 = 350/35 - 187/35 = 163/35
2 4/25 + 3 1/18 + 2 21/25 + 17/18 = 54/25 + 55/18 + 71/25 + 17/18 = 125/25 + 72/18 = 5 + 4 = 9