Question 4964
let train A travels at speed = 72 kph
let train  travels at speed = 120 kph
after 3 hours 2nd train i.e B train starts

so in 3 hour train a will cross = 3*72=216 km
the difference of speedof train A and train B = 120-72=48 kph

so to cover 216 km by the train B the time required = 216/48=4.5 hour
so after 4 hour and 30 minutes the train B will cross train A