recall your d = r*t or distance = rate * time
so... let's say B is going at a speed of "r"
A is slower than B by 20mph, or whatever "r" is, A's speed is r - 20
at a certain time "t", A runs 230 miles whilst B runs 330 miles, same "t" time
so.. hmmm

solve for "r", to see how fast B was going
what about A? well, A's speed is r - 20