1)
х + 320 = 80 * 7
х + 320 = 560
х = 560 - 320
х = 240
проверка:
240 + 320 = 80 * 7
560 = 560
2)
х - 180 = 20 : 3
х - 180 = 6 2/3
х = 6 2/3 + 180
х = 186 2/3
проверка:
186 2/3 - 180 = 20 : 3
6 2/3 = 6 2/3
3)
400 - х = 275 + 25
400 - х = 300
400 - 300 = х
100 = х
х = 100
проверка:
400 - 100 = 275 + 25
300 = 300