Пошаговое объяснение:
5/7х * 7/5 = 9/10
35/35х = 9/10
х = 9/10
3/х = 5/6 - 8/15
3/х = 25/30 - 16/30
3/х = 9/30
3/х = 3/10
х = 3 : 3/10
х = 3 * 10/3
х = 30/3
х = 10
