3/6 + 7/12 - 1/4 =
Numitor comun 12
6/12 + 7/12 - 3/12 = (6+7-3)/12 = 10/12 = 5/6
7/20 + 3/35 - 4/12 = 7/20 + 3/35 - 1/3
Numitor comun 2²·3·5·7=420
147/420 + 36/420 - 140/420 = (147+36-140)/420 = 43/420
15/75 - 3/25 + 4/20 = 1/5 - 3/25 + 1/5 = 2/5 - 3/25
Numitor comun 25
10/25 - 3/25 = (10-3)/25 = 7/25