Question 1321
 Missing conditions:

 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.