There are 60 minutes in an hour and 60 seconds in a minute.
60 * 60 = 3600
There are 3600 seconds in an hour.
3600/30 = 120
3600 seconds is 120 times 30 seconds.
If you go 1/2 mile in 30 sec, you go 120 times more distance in 1 hour.
1/2 mile * 120 = 60 miles
1/2 mile in 30 sec is the same as 60 miles per hour.