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 Cache and Ferry (How far can a truck go in a desert?)

A pick-up truck in the desert gets 10 miles per gallon of gas and starts
beside N 50-gallon gas drums, all full.  The truck's gas tank holds 10
gallons and is empty; the truck can carry only one 50-gallon drum.  How
far away from the starting point can you drive the truck?

[[Answer]]
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