You know 1.5 hours is equivalent to 90 minutes, and 90/10 is nine, which means that you can do nine times an amount in 90 minutes than you can in 10 minutes. So multiply 2.4 by 9 to get 21.6
Final Answer
If a person walks 2.4 miles in 10 minutes, he can walk 21.6 miles in 1.5 hours.