Let us assume that u is the average speed of airplane and t be the average time taken when airplane is travelling at u speed.
Speed is defined as the ratio of distance and time.
I.e, speed = distance/time
Therefore distance = speed * time
Distance = u*t
Now it is given that when airplane is travelling at u-60 kmph time taken is t+24 minutes.
When the speed is u+40 kmph time taken is t-12 minutes.
But the distance is the same in all three conditions.
So using the formula I mentioned above i.e., distance = speed*time
(u-60)*(t+24) = u*t
Therefore: 24u - 60t - 1440 = 0 ——(1)
(u+40)*(t-12) = u*t
Therefore: -12u + 40t - 480 = 0 ———(2)
Solving equations (1) and (2) will give you, u = 120 kmph.
Hope this helps.