S = 600/50 = 12 mps
S = 3600/12 = 300 sec = 5 min
This justifies what answer for what question? Q & A: 
user
A 600 meter long train crosses a signal post in 50 seconds. How long will it take to cross a 3 kilometer long bridge, at the same speed?
Options:
(A) 4 min
(B) 2 min
(C) 8 min
(D) 9 min
(E) 5 min
(E)
assistant