1/3х + 1/5х + 1/6х = 21/40
10/30х + 6/30х + 5/30х = 21/40
21/30х = 21/40
7/10х = 21/40
28/40х = 21/40
х = 21/40 : 28/40
х = 21/40 * 40/28
х = 21/28
1/4х + 1/6х + 1/8х = 39/56
6/24х + 4/24х + 3/8х = 39/56
13/24х = 39/56
х = 39/56 : 13/24
х = 39/56 * 24/13
х = (3 * 24)/56
х = 72/56 = 9/7 = 1 2/7