Two trains are moving in the same direction at the rate of 50 kmph and 80 kmph. The fast train crosses the slower train in 27 seconds. Find length of the faster train.

my answer is just a guess
relative speed between both trains = 30km/hr =(25/3)m/s
assume both trains r of equal length, so d+d=2d
t=27s
sub these values in d= s*t
2d= (25/3)*27 = 225 from which we get d=112.5m