Efren walks 4 miles per hour, and has a head start of 1/2 hour. So he's (1/2 of 4) = 2 miles out ahead when Gregory starts out.
How long does it take Gregory to gain the 2 miles that he's behind ?
Gregory runs at 8.5 mph, while Efren walks at 4 mph. Gregory gains (8.5 - 4) = 4.5 miles on Efren per hour. So it'll take him (2/4.5) = 4/9 of an hour to gain the 2 miles.
(2/9 hour) x (60 minutes/hour) = 26 minutes 40 seconds
