The Puzzler #25

This Month's Contest: WEIGHING THE COINS. You have seven stacks of coins, each with 100 coins. Each stack contains either only real coins or only phony coins. The goal is to figure out how many stacks of coins are authentic and how many are counterfeit.

This Month's Contest: WEIGHING THE COINS

You have seven stacks of coins, each with 100 coins. Each stack contains either only real coins or only phony coins. The goal is to figure out how many stacks of coins are authentic and how many are counterfeit. All the authentic coins weigh 10 grams each. Counterfeit coins weigh 11 grams each. Your only tool is an exquisitely accurate bathroom scale. What is the smallest number of weighings that can be used to determine how many stacks are made of real coins and how many are made of phony coins?

Please email your solution to John Kador [email protected] using the subject line “Weighing Coins.” Deadline is June 24, 2011. One entrant will be selected to receive a signed copy of John Kador's How to Ace the Brainteaser Job Interview. Good luck.

Brainteaser #2: BRANDS IN COMMON

What do the following American brands have in common?

Holiday Inn

7-Eleven

Budweiser

(Solution below.)

SOLUTION TO PREVIOUS PUZZLER: BIGGEST CHECK

To recap: There are three envelopes, each with a cashier's check for a random number of dollars. You can open any envelope and keep the check. Or you can discard that check and pick another envelope. If you're still not happy, you can pick the third envelope. But once you discard an envelope, you can't go back. What's your best strategy?

Solution: To optimize your odds of receiving the largest check, open an envelope at random, inspect it, and then whatever it is, discard it. Open another envelope. If this check is larger than the first, stop and cash your winnings. If the second check is smaller, discard it and go for the last envelope.

We received only about 45 entries; half had the correct solution. The winner, selected randomly from all correct responses, is Michael I. Chung, from RBC Wealth Management in Newport Beach, Calif. Chung created a probability table that showed how the optimum strategy improves one's chances of getting the biggest check from one-third to one-half. Congratulations to Michael. Good luck to all entrants for next time.

SOLUTION TO BRANDS IN COMMON: None of these so-called “American” brands is American, at least not any more: Holiday Inn: owned by Intercontinental Hotels, headquartered in England; Trader Joe's: owned and operated by a family trust of German billionaire Theo Albrecht; 7-Eleven: owned by Seven & I Holdings, a Japanese company; Budweiser: acquired by InBev, based in Belgium.; Toll House Cookies: owned by Nestle, a company based in Switzerland.

John Kador is the author of 10 books. His latest book is Effective Apology: Mending Fences, Building Bridges, and Restoring Trust (Berrett-Koehler). www.effectiveapology.com.

TAGS: Games