11/12-3/7 = (11*7 - 3*12) / (12*7) = 41/84
5/6-1/3= 5/6 - 2/6 = 3/6 = 1/2
1/2-1/14= 7/14 - 1/14 = 6/14 = 3/7
11/18-2/9= 11/18 - 4/18 = 7/18
9/20-11/36= (9*36 - 11*20) / (36*20) = 104/720 = 13/90
8/15-1/12= (8*4 - 1*5) / (3*4*5) = 27 / 60 = 9/20
1/4-1/12= 3/12 - 1/12 = 2/12 = 1/6
8/21-3/8= (8*8 - 3*21) / (21*8) = 1/168
5/7-1/6= (5*6 - 1*7) / (6*7) = 23/42
8/15-1/5=8/15 - 3/15 = 5/15 = 1/3
1/5-1/15=3/15 - 1/15 = 2/15
5/6-3/5= (5*5 - 3*6) / (5*6) = 7/30
5/9-4/12= (5*4 - 4*3) / (3*3*4) = 8/36 = 2/9