A walk at 4 km/hr. Three hours after A started, B cycles at 20 km/hr towards A.
How far from the start does B catch up with A?
Let x be the distance in which B catches up with A.
Now time taken by A will be 3 hours more than B.
x/4 = x/20 + 3
x/4 - x/20 = 3
4x/20 = 3
x/5 = 3
x = 15 km