[Dirty Meg]

There is a water tank. You have a five pint container and a three pint container. With no wastage, you must draw 4 pints of water from the tank. You can't estimate. You do not know how much is in the water tank. How can you do this?

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answer
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Fill the five pint container. Fill the three pint container from the five pint container, leaving two pints in the five pint container. Empty the three pint container into the water tank. Pour the two pints from the five pint container into the three pint container. Fill the five pint container, and then top up the three pint container (which now contains two pints) from the five pint container, leaving four pints in the five pint container.