Distance = speed × time is the whole topic. What varies is which of the three is constant: in a "same distance" question, speed and time are inversely proportional, and that single observation answers a surprising number of questions without any algebra.
Everything involving two moving things - trains passing, a boat in a current, someone catching someone up - is relative speed. Add the speeds when they move towards each other, subtract when they move the same way.
Try each one before opening the answer. A solution you have read is not a solution you can produce under time pressure, which is the only kind that counts.
Q1 A man covers a distance at 40 km/h and returns at 60 km/h. What is his average speed for the whole journey?
48 km/h.
Not 50. Equal distances mean the harmonic mean: 2 × 40 × 60 / (40 + 60) = 4800/100 = 48. Check it with a number - 120 km each way takes 3 hours out and 2 back, so 240 km in 5 hours, which is 48.
Q2 A 150 m train travelling at 72 km/h crosses a platform in 20 seconds. How long is the platform?
250 m.
Convert first: 72 km/h × 5/18 = 20 m/s. In 20 seconds it covers 400 m, and that distance is the train plus the platform. So the platform is 400 − 150 = 250 m.
Q3 Two trains 120 m and 180 m long run towards each other at 42 km/h and 30 km/h. How long do they take to pass completely?
15 seconds.
Approaching, the speeds add: 72 km/h, which is 72 × 5/18 = 20 m/s. The distance to clear is both lengths, 300 m. Time = 300/20 = 15 seconds.