**Given information:**
- Length of the train = 100 m
- Speed of the man = 5 km/hr
- Time taken to cross the man = 7.2 seconds

To find the velocity of the train, we can use the formula:

Velocity = Distance / Time

**Conversion:**
Before we calculate the velocity, let's convert the speed of the man from km/hr to m/s, since the length of the train is given in meters and time is given in seconds.

1 km/hr = 1000 m/3600 s = 5/18 m/s

So, the speed of the man is 5/18 m/s.

**Calculating the relative velocity:**
When two objects are moving in opposite directions, their relative velocity is the sum of their individual velocities.

Relative Velocity = Velocity of the train + Velocity of the man

Since the velocity of the man is in the opposite direction, it will be negative.

Relative Velocity = Velocity of the train - Velocity of the man

**Calculating the relative distance:**
The relative distance is the sum of the length of the train and the distance covered by the man during the time taken to cross the train.

Relative Distance = Length of the train + Distance covered by the man

The distance covered by the man can be calculated using the formula:

Distance = Speed × Time

**Calculating the velocity of the train:**
Now, we can substitute the values into the formula for velocity:

Velocity = Relative Distance / Time

Substituting the values and solving the equation will give us the velocity of the train.

**Final Answer:**
After calculating the velocity using the given values, the velocity of the train will be the final answer.

According to question, a 100 m long train ( to change km ) = 0.1 km ; and a man travelling opposite direction then relative velocity is ( x + 5 ) . and take 7.2 sec ( to change in hour ) = 0.002 hour . we know that , time = distance / speed => 0.002 = 0.1 / x+ 5 = x + 5 = 0.1/0.002 = x+5 = 50 = x = 45 ( 50 - 45 ) km/ h.....