Clearly you lot on here need my book, Diversion Planning.....!! Got a few useful things about speed, distance and time. Near as need be, groundspeed is 100 kts, which is 5/3 of a mile per min.....multiply by the number of minutes and theres your answer. (25/6 miles or approx 4 and 1/6 miles, the approximation coming from the round up from 98 to 100 kts. In a light aeroplane you wont want to be this accurate, for the most part!! 5/3 per min is 10/6 per min, and 2.5 times this is 25/6. See, no calculator!)