To answer this we must determine how many 10min segments there are in 1.5 hours. 1.5 hours would equal 90 mins (60mins in 1 hour, and 30mins in 0.5 of an hour).
90 mins divided by 10 mins = 9. (there are 9, 10 minute segments in 1.5 hours)
Therefore, if a person can bike 2.4 miles in 10 mins, they can bike 21.6 miles in 90 minutes (2.4 x 9 = 21.6)