A train crosses a man standing on a platform in 7 seconds and crosses the platform of length 140 m in 21 seconds. Find the speed of the train.

Irza Afzad Arithmetic/ Basic Math, css, Full List, Mathematics

A. 20 m/s
B. 22 m/s
C. 25 m/s
D. 36 m/s

Solution:
Length of train = speed × 7.
Length of train + 140 = speed × 21.
⇒ 21v – 7v = 140 → 14v = 140 → v = 10 m/s = 36 km/h.