If a person bikes 2.4 miles in 10 minutes how far can he bike in 1.5 hours

There are 60 minutes in 1 hour.
1.5 hours is (1.5 x 60 minutes) = 90 minutes

In order to bike for 90 minutes, the person
has to bike for 10 minutes 9 times.

IF he can keep going at the same rate for 9 times as long,
then he would cover  (9 x 2.4 miles) = 21.6 miles.

But that's a big 'IF'.  People usually start to slow down
as they keep going for longer and longer time.