BEagle,
If the groundspeed is 60 knots then 21nm will take 21 minutes.
If the groundspeed is 70 knots then it will take 6/7 of the time i.e. 18 minutes
If the groundspeed is 80 knots then it will take 6/8 (3/4) of the time.
If it is 75 knots then it is half way between.
Regards,
DFC