Henry must travel from A to B and he plans to go at a certain speed

Let d be the distance between A and B.

Let x be the planned speed in km/hr and let t be the time it takes in planned speed to reach B in hrs.

Therefore, d = xt     (1)

Now, speed is increased by 5km/hr (i.e. x+5) and time is reduced by 5 hrs (t-5)

Therefore, d = (x+5)(t-5)    (2)

Now, speed is increased by 10km/hr (i.e. x+10) and time is reduced by 8 hrs (t-8)

Therefore, d = (x+10)(t-8)     (3)

From 1, 2 and 3

xt = (x+5)(t-5) = xt - 5x + 5t - 25

So, x = t-5

Now, xt = (x+10)(t-8) = xt - 8x + 10t - 80

So, 8x = 10t - 80

Substitute x = t-5 in above equation, 

8t - 40 = 10t - 80

t = 20

Therefore, x = t -5 = 15 km/hr

Correct option is 15km/hr