320-(bx4+120):5=40x6

320-(b*4+120):5 =40*6

(4b+120):5  = 320-240

4b+120 = 5*80

4b = 400-120

4b = 280

b = 280:4

b = 70

Проверяем: 

320-(70*4+120):5 = 320-(280+120):5 = 320-80 = 240