65 mph means in 1 hour you drive 65 miles.
To find out how many feet you drive in 1 second you need to convert the above in terms of feet and secs
1 hour = 60 minutes and 1 minute has 60 secs
So 1 hour has 60* 60 secs = 3600 secs.
1 mile - 5280 feet
65 miles = 65*5280 feet
So in 3600 secs you are driving 65 * 5280 feet
