Question 1079488
I do not know the distance from Modhupur to Kolkata, but I do not need it to solve the problem.
{{{f}}}= the ratio of the speed of the faster train to the speed of the slower train.
So,
1) in the same length of time, the faster train travels {{{f}}} times more distance, and
2) to cover the same distance it takes the slower train {{{f}}} times longer time.
 
The distance the slower train had covered when they passed,
was later covered by the faster train in {{{1}}} hour.
It must have taken the slower train {{{f}}} hours to cover that distance.
Up to that point the faster train had covered {{{f}}} times as much distance
so the slower train has {{{f}}} times farther to go than it already covered.
It will take it {{{f}}} times as long, or {{f*f=f^2}}} hours.
Since we are told it takes it {{{4}}} hours,
{{{f^2=4}}} ---> {{{highlight(f=2)}}} .