It took him 1 hour and 30 mins .
He needs to go 15 miles. It takes him 1 hour to get 10 miles. After 1 hour has passed he still needs 5 more miles to go. 5 miles is half of 10 so half of 1 hour is 30 mins. 1 hour + 30 mins is 1 hour and 30 mins. (It took him 1 hour 30 mins)