Here's one of several ways to figure out the
answer: Grandma walked 1 mile up the hill in 30
minutes and 1 mile down in 15 minutes. In other
words, 1 mile took her 45 minutes round trip.
The whole trip took her 6 hours, or 360 minutes;
if you divide the overall time (360 minutes) by
the time it took to do 1 mile (45 minutes), you
find out that the overall length of the hill is
STRATEGY: Make it simpler.
Source: Inspired by a puzzle
in The Adler Book of Puzzles and Riddles, or Sam
Loyd Up-to-Date, Irving and Peggy Adler. New York:
John Day, 1962.