A boy, a girl and a dog go for a 10 mile walk. The boy and girl can walk at 2 mph and the dog can trot at 4 mph.
They also have a bicycle which only one of them (including the dog!) can use at a time.
When riding, the boy and girl can travel at 12 mph while the dog can pedal at 16 mph.
What is the shortest time in which all three can complete the trip?
2.75 hours (decimals are a little easier to follow)
Girl rides 5.4 miles in 0.45 hours; leaves bike; walks 4.6 miles in 2.3 hours.
Dog trots 5.4 miles (to location of bike) in 1.35 hours; takes bike BACK 0.8 miles (to 4.6-mile mark) in 0.05 hours; then trots forward the remaining 5.4 miles in 1.35 hours.
Boy does same as girl but in reverse order: walks 4.6 miles in 2.3 hours; picks up bike; rides 5.4 miles in 0.45 hours.
I always use Excel to solve this kind of thing. It allows me to "compartmentalize" the mathematical relationships -- so I don't have to think about everything at once!
Edited on February 3, 2004, 2:42 pm
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