Question

Two trains 100 meters and 120 meters long are running in the same directions with speed of 72 km/hr and 54 km./hr. in how much time will the first train cross the second?

Answer 1

Since you haven't given the position of trains, I am assuming that they are as in above image.

Change their velocities to m/s.

Now, relative velocity of A wrt B is 20 - 15 = 5 m/s.
relative distance between A and B is 120 m

time taken by A to cross B will be 120/5=24 seconds.

if the distance is given in ques, kindly make necessary changes in procedure.

Answer 2

that is simple supposed at both the train start there journey from a similar platform .
then the difference of velocity on them is 18(72-54) km/h ...
$its \: means \: that \: the \: train a \: can \: go \\ \: ahead \: 18km \: to \: train \: b \: in \: 1h \:$
but we will have to cross only 120 m so the train a i will take
$1h = 18km \\ 1km = 3600 \div 18000 = 0.2sec×120= 24$
I hope it will help u plzz mark me as brilliant