С * 4 = 1/5
с= 1/5 : 4 = 1/5 : (4/1) = 1/5 * 1/4
с = 1/20
проверим:
1/20 * 4 = 1/5
4/20 = 1/5
1/5 = 1/5
а - 4/9 = 1/3
а= 1/3 + 4/9 = 3/9 + 4/9
а= 7/9
7/9 - 4/9 = 1/3
3/9 = 1/3
1/3 = 1/3
у: 3 = 2/7
у/3 = 2/7
7у = 2*3
7у = 6
у= 6/7
6/7 : 3 = 2/7
6/7 * 1/3 = 2/7
6/21 = 2/7
2/7 = 2/7