Did you know that if you carry a certain combination of coins, you can always have the correct change on you, for any amount between one cent and $.99? The fewest number of coins you can carry to do that is ten, consisting of three quarters, two dimes, one nickel, and four pennies.

Here's a project to make a holder so you can carry this combination easily inside your wallet, and it's about the size of a credit card. Instructables shows you how to take a thin piece of cardboard and turn it into an exact change carrier. Simple but neat. – Charlie White

Always have Correct Change in your wallet [Instructables, via boingboing]

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