Subtract each number from the mean (exp: 8-12=-4) and add all the absolute values (the numbers regardless of positive or negative (4 is the absolute value of -4)) together, then divide by the number of numbers (15) that gives you 2.4 :)
to get to 45mins you have to multiply 15 min by 3 to get 45mins. If you do it on the bottom you do it on the top. so you then multiply 2miles by 3 to get 6miles. So he travels 6 miles in 45 mins.