5/7 * (21/20 - х) + 16/21 : 8/7 = 1 1/4
5/7 * (21/20 - х) + 16/21 * 7/8 = 5/4
5/7 * (21/20 - х) + 112/168 = 5/4
5/7 * (21/20 - х) + 14/21 = 5/4
5/7 * (21/20 - х) + 2/3 = 5/4
5/7 * (21/20 - х) = 5/4 - 2/3
5/7 * (21/20 - х) = 15/12 - 8/12
5/7 * (21/20 - х) = 7/12
21/20 - х = 7/12 : 5/7
21/20- х = 7/12 * 7/5
21/20 - х = 49/60
- х = 49/60 - 21/20
- х = 49/60 - 63/60
- х = - 14/60
х = 14/60
х = 7/30
проверка:
5/7 * (21/20 - 7/30) + 16/21 : 8/7 = 1 1/4
5/7 * (63/60 - 14/60) + 16/21 * 7/8 = 1 1/4
5/7 * 49/60 + 112/168 = 1 1/4
245/420 + 14/21 = 1 1/4
49/84 + 56/84 = 1 1/4
105/84 = 1 1/4
1 21/84 = 1 1/4
1 1/4 = 1 1/4