Explanation

3/7 mile = 10 min. 10/3 = 3.334 min = 1/7 mile. 3.334 x 7 = 23.334 min, = 1 mile 23.334 min. - 10 min. = 13.334 min. or 13 1/3 min. 60 sec./3 = 20 sec. 

It will take 13 minutes and 20 seconds to walk the rest of the mile.