Question 201114
Assume I have a stopwatch. I'll start
it when the 2nd train leaves at 2 PM.
How far has the 1st train gone in 
2 hours?
{{{50*2 = 100}}} miles
When I start the stopwatch, both trains
will travel for {{{t}}} hrs
If the 2nd train's distance is {{{d}}},
the 1st train's distance is {{{d - 100}}}
Now I can write an equation for each one
2nd train:
{{{d = 90t}}}
1st train:
{{{d - 100 = 50t}}}
Substituting:
{{{90t - 100 = 50t}}}
{{{40t = 100}}}
{{{t = 2.5}}}
This is the elapsed time starting at 2 PM
The 2nd train catches the 1st train at 4:30 PM