If it travels 20 metre in one second then it travels 5*20 metre every 5 seconds
That means it travels 100 metre every 5 seconds and 1000m or 1km every 50 seconds
Now by unitary methods 1*72 = 72 km and 50*72 = 3600 seconds
There fore it travels 72km per 3600 seconds or 1 hour
