Question 836674
They are traveling toward eachother so their combined rates added are how fast they approach eachother.


Let d = distance between them at start
F = fast rate train
S = slow rate train
Let x = unknown time when they meet, variable to solve.


The two trains meet when the sum of their distances traveled equals d.  A data table might help:


_______________speed__________time__________distance
Fast Train_____F______________x_____________Fx
Slow Train_____S______________x_____________Sx
Total________________________________________d


Equation formable, {{{highlight(Fx+Sx=d)}}}
{{{(F+S)x=d}}}
{{{highlight(highlight(x=d/(F+S)))}}}
That is the formula after solving symbolically.


Substitute your known or given values to find the value for x.