An hour is 60 minutes.
3/4 of an hour is 3/4 of 60 minutes which is 3/4 * 60 minutes = 45 minutes.
(45 minutes)/(15 minutes) = 3
45 minutes is 3 times 15 minutes, so in 45 minutes he hikes 3 times the distance he hikes in 15 minutes.
3 * 1/2 mile = 3/2 mile = 1 1/2 miles or 1.5 miles