Question

Two trains are traveling towards each other, starting 100 kilometers apart. One train travels at 40 kilometers per hour while the other travels at 60 kilometers per hour. The plane starts flying at 90 kilometers per hour, starting from the location of the faster train. Once the plane reaches the slow train, it turns around immediately and flies back. Once it reaches the fast train again, it turns around and flies towards the slow train, etc.
When the trains pass each other, how far will the plane have flown?

Answer 1

the trains are closing at 100 kph
... so it takes them 1 hr to pass

the plane flies 90 kph for 1 hr