BTGmoderatorRO wrote:A train traveling at 72 kmph crosses a platform in 30 seconds and a man standing on the platform in 18 seconds. What is the length of the platform in meters?

A.240 meters
B.360 meters
C.420 meters
D.600 meters
E.Cannot be determined

OA is A

I got my answer in km, how do I do a proper conversion?

We can let p = the length of the platform, in meters, and t = the length of the train, in meters.

When we say the train crosses a platform in 30 seconds, it really means it takes 30 seconds for the nose of the train to enter one end of the platform and the rear of the train to exit the other end of the platform. Thus, in 30 seconds, not only does the train's nose travel the entire length of the platform but also its entire body length does. Thus, we have (notice that 72 kmph = 72000 meters per hour, and 30 sec = 30/3600 = 1/120 hr):

72000 x 1/120 = p + t

600 = p + t

p = 600 - t

We are also given that the train crosses a man (whose body width is negligible) standing on the platform in 18 seconds. So when the train crosses him, it only travels its body length in 18 seconds. Thus we have (notice that 18 sec = 18/3600 = 1/200 hr):

72000 x 1/200 = t

360 = t

Therefore, the platform has a length of 600 - 360 = 240 meters.

Answer: A