Question 668372
Remember that d = r*t  (distance is equal to rate times time).
When the second train overtakes the first, they will have traveled the exact same distance. Therefore, we can set the rate of the first train (40 mph) times the time of the first train (t) equal to the rate of the second train (80 mph) times the time of the second train (t - 2)
The second train's time is represented by t - 2 because it will have been travelling for 2 hours less than the first train when it catches up.
40t = 80(t - 2)
Now, we can distribute the 80 to the parentheses:
40t=80t-160
Then, we can subtract 80t from both sides:
-40t=-160
Finally, we divide both sides by -40
t=4
So we know that the first train will have been travelling for 4 hours before the second train overtakes it.