1) 25/28 - 2/21 = 67/84
2) 41/54 - 35/72 = 59/216
3) 20/21 - 3/14 = 31/42
4) 40/143 + 39/110 = (НОК=1430) = (40*10+39*13)/1430 = (400+507)/1430 = 907/1430
5)42/140 +41/150 =(НОК=2100) = (43*15+41*14)/2100 = (645+574)/2100 = 1219/2100
6) 73/80 - 37/100 = 217/400