Пошаговое объяснение:
a)
1) 21/20 - 7/30 = 63/60 - 14/60 = 49/60
2) 5/7 * 49/60 = 7/12
3) 16/21 : 8/7 = 16/21 * 7/8 = 2/3
4) 7/12 + 2/3 = 7/12 + 8/12 = 15/12 = 1целая 3/12 = 1цел. 1/4
ответ: 1цел.1/4
б)
1) 12/25 - 4/15 = 36/75 - 20/75 = 16/75
2) 4/45 : 16/75 = 4/45 * 75/16 = 5/12
3) 15/16 * 4/15 = 1/4
4) 5/12 + 1/4 = 5/12 + 3/12 = 8/12 = 2/3
ответ: 2/3
