80% = 80/100 = 4/5 - сократили на 20
4/7 * х = 4/5 * 70
4/7 * х = 4 * 14
4/7 * х = 56
х = 56 : 4/7
х = 56 : 4 * 7
х = 98
Проверка:
4/7 * 98 = 4/5 * 70
4 * 14 = 4 * 14
56 = 56
