Question 551216
let speed of B be x mph
in 2 hours before A starts B  has already traveled  2x miles

So the difference between A & B when B starts is 2x miles ( d= rt)
A's rate is 20 km/h more than B's rate . 
catchup speed = 20 mph
Catch up distance is 2x miles
t=d/r
A catches up in 4 hours

2x/20=4
2x=80
x=40

speed of B is 40 mph
speed of A = 60 mph