] The item I had just bought cost 29 cents. I gave the
] cashier a dollar bill, and she gave me two quarters, two
] dimes, and a penny in change. She could just as well have
] given me seven dimes and a penny or some other
] combination of coins adding up to 71 cents, but there's
] no way she could have made change with fewer than five
] coins.
]
] Most businesses in the United States make change using
] just four different types of coins: 1 cent (penny), 5
] cents (nickel), 10 cents (dime), and 25 cents (quarter).
] This distribution of coinage suggests an interesting
] question: Is it the most efficient way to make change? In
] other words, is this the optimal choice of coin values
] for minimizing the number of coins required to handle
] typical transactions?
]
] Computer scientist Jeffrey Shallit of the University of
] Waterloo has worked out an answer. In the current issue
] of the Mathematical Intelligencer, he contends that "what
] the U.S. needs is an 18-cent piece."

Math Trek : Coins for Making Change Efficiently, Science News Online, May 10, 2003