Question 980571
The two trains moving toward each other are approaching at the sum of their individual rates.


RT=D relates rate, time, distance.


r, the speed or rate of the slower train.
Slow train, r
Fast train, r+20
time, 2 hours
distance covered, 500-300=200


{{{(r+r+20)*2=200}}}
If this equation makes sense to you, then just simple arithmetic should be apparent for how to solve for r.


ONE SOLUTION for r;
not two solutions.




----

Your teacher would have meant that if the FASTER train goes at speed of r, then the SLOWER train goes at the speed of r-20.  The result for rates of fast and slow trains will still be the same as if you assigned r to the slow train and r+20 to the fast train.