A takes 4 h more than the time taken by B to walk D km. If A doubles his speed, he can make it in 2 h less than that of B. How much time does B require for walking D km?

Let B takes x h to walk D km.

Then, A takes (x + 4) h to walk D km.

With double of the speed,

A will take \( \Large \frac{1}{2} \left(x+4\right) h \).

According to the question,

\( \Large x - \frac{1}{2} \left(x+4\right) = 2 \)

\( \Large 2x - \left(x+4\right) = 4 \)

\( \Large 2x - x - 4 = 4 \)

Therefore, \( \Large x = 4 + 4 = 8 h \)