# The Puzzler #16

THIS MONTH'S CONTESTPRIME 907 The number 907 is prime. It has no factors other than one and itself. To prove that 907 is prime, you must check to see that it not divisible by any other number, beginning with 2, 3, 5, 7 and so on. The challenge: How far do you need to go up the number line before you can conclude that 907 is in fact prime? Please email your solution to John Kador ([email protected])

### THIS MONTH'S CONTEST PRIME 907

The number 907 is prime. It has no factors other than one and itself. To prove that 907 is prime, you must check to see that it not divisible by any other number, beginning with 2, 3, 5, 7 and so on. The challenge: How far do you need to go up the number line before you can conclude that 907 is in fact prime?

Please email your solution to John Kador ([email protected]) using the subject line “Prime 907.” Deadline is October 26, 2009. Two accurate responses will be selected to receive a signed copy of John Kador's How to Ace the Brainteaser Job Interview. Creativity and elegance increase one's odds. Good luck.

 3 2 12 1 4 8 2 4 ??? Solution at bottom.

### SOLUTION TO PREVIOUS PUZZLER: HEADS & TAILS

To recap: You are blindfolded. On the table in front of you are a number of ordinary coins. You are told that exactly 26 coins show heads. Your challenge (while blindfolded): Divide the coins into two sets so that the same number of coins in each set show heads.

We received only 15 responses to this puzzle. Only four got the right answer. (Standing coins on their edges is cute, but it doesn't address the challenge.) Solution: Select 26 coins at random to serve as Set A. Call all remaining coins Set B. Now, simply turn all the coins in Set A over. Sets A and B now have the same number of coins showing heads. Many people doubt that the solution is this simple. But it is. Try the experiment with any configuration of heads and tails and you'll find it works.

Congratulations to this month's winners, selected at random from the four who sent in correct responses: Chadwick D. Richards, Financial Advisor, Waddell & Reed, Inc. Nashville, TN and John J. Skittone, First Vice President, Investments, Weis/Skittone Wealth Management Group of Wells Fargo Advisors, Lake Forest, IL. They will receive signed copies of my book.

CORRECTION: The solution to the Candle Time problem as published in the last issue was not accurate, as a number of readers pointed out. My apologies for confusing an already difficult puzzle. For the record here's the solution: At the same instant, light both ends of Candle A and one end of Candle B. When Candle A has burned through, light the other end of Candle B. When Candle B has finished burning, it will mark 45 minutes.

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

SOLUTION TO WHAT IS THE MISSING NUMBER?: 16. Multiply the first two numbers in each row and then double the product.