The problem did not have enough details.It seems like you want to know that !st person leaves and 2nd person leaves; how long before they catch up together? These types of problems are called Uniform Motion or Rate-Time-Distance.There is a good video for you to watch.Similar to the problem you need help with. Yourmathgal channel on Youtube,she is a teacher.Have watched a lot of her videos.Watch the video on her channel called "Rate-Time-Distance Intro".It is 10:40 and she explains the 6 kinds of Uniform Motion problems.