First things first, distance = speed x time
So let's have "t" represent the time by both trains.
The first train shall be Train A and the other becoming Train B, so now we have Train A going 80t miles and Train B going 100t miles.
If two trains head toward each other, they will have traveled a total of 252 miles. The faster train will have traveled further (Train B), so the meeting point will be closer to the slower train's starting point (Train A).
So now we do 80t + 100t = 252
Which becomes 180t = 252
And by diving 252 by 180, you'll get that t is 1.4
This would mean they would meet at one hour and 24 minutes.