You can put this solution on YOUR website!
Unless there is some information to have the difference of their apart at
some time, we cannot solve this problem.
Usually,the time of catching up = D(distance) / (V2 - V1)
= D/(55 -20) = D/35.
Also, this is a pretty easy question. I hope you can solve it by
yourself without posting again after read this hint and get the enough
information about the distance.