25/28 - 2/12=25/28 - 1/6=75/84 - 14/84=61/84
41/54 - 35/72=164/216 - 105/216=59/216
(54=18*3, 72=18*4, общий знаменатель-=18*3*4=216)
20/21 - 3/14=40/42 - 9/42=31/42
40/143 - 39/110=4400/15730 - 5577/15730= - 1177/15730
43/140 - 41/150=645/2100 - 574/2100=71/2100
73/80 - 37/100=730/800 - 296/800=434/800=217/400