If you have to cover 21 miles in 21 minutes, then you need to cover 1 mile every minute.
There are 60 minutes in an hour, so if you kept that up for an hour, you'd cover 60 miles.
At 60 mph, you can drive 10 miles. At 30 mph, you can drive 5 miles.
About 19 minutes
Five minutes at 60 mph
115.4 minutes.
16 minutes at 60 mph
19.5 miles.
148.75 miles.
54 minutes.
15 minutes.
36 minutes.
About 35 miles.
7.67 miles.