How long will a passenger sitting at the window of a train traveling at a speed of 72 km / h

How long will a passenger sitting at the window of a train traveling at a speed of 72 km / h see an oncoming train passing by him at a speed of 36 km / h and having a length of 150 m

72 km / h = 20 m / s;
36 km / h = 10 m / s;
We consider:
1.20 + 10 = 30 m / s – at this speed they approach each other;
2. 150/30 = 5 s – the passenger will see a passer-by;
Answer: Within 5 seconds, a person sitting by the window will see a passer-by through the window.