Question

Two trains 321 m and 99 m in length are running in opposite directions with velocities of 40 kmph and 32 kmph. In what time will they completely cross each other?

Answer 1

Relative velocity, $v=(40+32)kmph=72kmph=72\times \frac{5}{18}m{s}^{-1}=20m{s}^{-1}$
Total distance covered, $d=(321+99)m=420m$
So, time taken, $t=\frac{d}{v}=\frac{420}{20}=21s$