а) (10+15)*2 = 25*2 = 50

90 - (10+15) *2 = 90 - 25*2 = 90-50 = 40

б) 90 - 40 = 50

(90 - 40) : 2 = 50 : 2 = 25

(90 - 40) : 2 - 10 = 50 : 2 - 10 = 25 - 10 = 15