A train 100 m long travelling at 40 m/s starts overtaking another train 200 m long travelling at 30 m/s. The time taken by the first train to pass the second train completely is:

Since the trains are moving in the same direction, their relative velocity is the difference of their speeds, which gives 40 m/s minus 30 m/s equal to 10 m/s. The total distance to be covered is the sum of their lengths, giving 100 m plus 200 m equal to 300 m. Using the time formula where time equals distance divided by speed, the time taken is 300 m divided by 10 m/s, which results in 30 s.

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