4/7 + 2/9 = 36/63 + 14/63 = 50/63
4/15 + 13/20 = 16/60 + 39/60 = 55/60 = 11/12
19/21 + 1/15 = 95/105 + 7/105 = 102/105 = 34/35
1/8 + (19/21 + 2/5) = 1/8 + (95/105 + 42/105) = 1/8 + 137/105 = 105/840 + 1096/840 = 1201/840 = 1 361/840
1/8 + 1/12 + 3/8 + 5/24 = 3/24 + 2/24 + 9/24 + 5/24 = 19/24
1/8 + (5/9 - 1/4) = 1/8 + ( 20/36 - 9/36) = 1/8 + 11/36 = 9/72 + 22/72 = 31/72