Platfrom 9 and 3/4's

Say train X leaves station A for Station B at 9am and train Y leaves station B for Station A at the same time; at what time will both trains intersect if they are following the same route measuring 100km.

Additional info: Train X is travelling at 50km/h and Y at 20km/h

If the distance a train travels is known as M distance:
M / the km travelled within an hour = the time required to attain M distance

Hence,

The point at which both trains intersect would be:

M/50 = (100-M)/20

2M = 5(100-M)

7M = 500

M = 71.42km

So at what time did both trains cross?
Train X would take T time to travel 71.42km

71.42/50 = 1.428 hours

Hence the trains would cross at:

9am + 1.428 hours = 10.25am

In the eventuality that train X and Y leave at different times, the position of the train that leaves first would have to be quantified first. Using the above example, if train Y instead left at 10am, what time did both trains intersect?

At 10am, train X which left at 9am would have finished travelling 50% of the track. And hence instead of 100-M, the formula should be edited to 50-M. Meaning:

M/50 = (50-M)/20

2M = 5(50-M)

7M = 250

M = 35.714

So Train X would intersect train y after travelling a total of 85.714km. And it would have taken T time to attain this distance:

85.71/50 = 1.714 hours

Meaning both trains would intersect at 9am + 1.714 hours = 10.42am

This is right, right? I never did any physics or anything in highschool, but I decided to try solve this problem since I was thinking about it.

Comments please Physics students!