Hi,

Suppose B takes t hours to walk d km.

Then, A takes (t + 2) hours to walk d km.

With double speed, A will take 21(t+2) hours.

∴t−21(t+2)=1⇒2t−(t+2)=2⇒t=4

∴ B takes 4 hours to walk d km.

OR

Let time take by B to walk D km = T

Time taken by A to walk D km = (T +2).

Since, ST = D. When distance is constant, Speed is inversely proportional to time taken, means if speed become double, time taken would become half.

So,

In second case time taken by A = (T +2)/2.

Now, this time A takes 1 hr less than B.

(T +2)/2 = T-1

T +2 = 2T - 2

T = 4 hours.

Hope this helps.