MPH = Miles per Hour
MPM = Miles Per Minute
There are 60 minutes in an hour.
So travelling n miles/hour is equivalent to travelling n miles/60 minutes.
Dividing both the numerator and denominator by 60 yields:
n/60 miles per 60/60 minute
= n/60 Miles per Minute
To calculate miles travelled by plane, simply multiply the speed by time as indicated by the formula you given ( d = s x t )