Given:
The speed of the sound is
![S=1.2\cdot10^3\frac{\operatorname{km}}{h}]()
To find the time for sound to travel 7.2*10 km
Speed = distance over the time
So, time will be the distance over the speed
so, the time will be:

Now, convert from hours to minutes
1 hour = 60 minutes
so, the time will be:

So, the answer will be: time = 3.6 minutes