Girl, boy and dog
Found this one on Maths is Fun.
A girl, a boy and a dog are out for a walk. The girl walks at a constant speed of 4 miles per hour, and the boy at a constant speed of 3 miles per hour. The dog, having lots of energy, runs at a constant 7 miles per hour. The dog runs back and forth between the boy and the girl. He looses no time changing direction. After one hour of this, how far as the dog traveled?
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