Lets t be the time that needed for trains to be 550 mi apart.
1st train will go (60*t) mi
2nd train will go (50 *t) mi
Altogether
(60t +50t), at the same time this distance equals 550 mi.
So, we can write
60t +50t =550
110t = 550
t=550/110
t=5 h
We have now proven that it would take 5 hours for the two trains to be 550 mile apart.
Hope this would help you.!!:)