5) 12/27 + 14/21 = 12*7/27*7 + 14*9/21*9 = 84/189 + 126/189 =
= (84+126)/189 = 210/189 = 10*21/9*21 = 10/9 = (9+1)/9 =
= 9/9 + 1/9 = 1 +1/9 = 1 1/9
6) 14/24 - 15/40 = 14*5/24*5 - 15*3/40*3 = 70/120 - 45/120 =
= (70-45)/120 = 25/120 = 5*5/24*5 = 5/24
7) 12/18 - 5/60 = 12*10/18*10 - 5*3/60*3 = 120/180 - 15/180 =
= (120-15)/180 = 105/180 = 7*15/12*15 = 7/12
8) 4/24 + 3/36 = 4*3/24*3 + 3*2/36*2 = 12/72 + 6/72 =
= (12+6)/72 = 18/72 = 1*18/4*18 = 1/4