ANSWER:C
Explanation:
Speed of goods train = 165km/hr
length of mail train (P) = 750m
length of goods train (Q) = 700m
Alternately, we can directly use the formula:
Time = ( p+q) / (v1+v2) sec
(here P and Q are length of trains and V1 and V2 are speeds of two trains)
Goods train and the mail train move in opposite direction.
Hence, the relative speed is the addition of two speeds.
Convert 165 km/hr into m/s
165 km/hr = (165*5/18)m/s
= 45.833 m/s
Therefore,
Time = ( p+q) / (v1+v2) sec
30 = (750+ 700) /(45.833+ v2)
= 1450 / 45.833 +v2
45.833+v2 = 48.333
V2 = 2.5 m/s
Therefore the speed of the mail train is 2.5 m/s