Black and White

By Philip Hamilton Williams, Evening News and Post, 1895. White to mate in two moves.

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1. Qb1 and every Black move loses.

August 17, 2012February 6, 2013 | Puzzles

Up From Down

Mount Everest rises 29,029 feet above sea level, and Ecuador’s volcano Chimborazo rises only 20,702 feet. But because Earth bulges at the equator, Chimborazo is actually farther from the center of the planet. If we could connect the two peaks with a water pipe, in which direction would the water flow?

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Sea level gives us a convenient equilibrium water level. Since Everest rises higher, the water would flow toward Chimborazo.

August 12, 2012February 6, 2013 | Puzzles

Black and White

pierce chess problem

By W. Timbrell Pierce, 1873. White to mate in two moves.

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1. Qh5 Ke4 2. Rxc4#. (If the knight moves then the queen can mate immediately from e5.)

August 10, 2012February 6, 2013 | Puzzles

Spin City

Imagine two concentric roulette wheels, each divided into 100 sectors. Choose 50 sectors at random on each wheel, paint them black, and paint the rest white. Prove that we can now position the wheels so that at least 50 of the aligned sectors match.

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Follow a sector on the inner wheel through a complete revolution: That sector will find exactly 50 matches. The same is true for each of the 100 sectors on the wheel; altogether, as the wheel turns through 100 positions, there will be 100 × 50 = 5,000 matches. This means that the average number of matches per position is 50, so a position must exist with at least 50 matches.

August 9, 2012February 6, 2013 | Puzzles

Horse Play

horse play 1

A carnival worker is asked to paint the deck of a carousel. Because the center of the carousel is occupied by machinery, he can’t measure its diameter or even its radius. The best he can do is to take the measurement shown in green, which is 42 feet.

He’s explaining this apologetically when his supervisor stops him. “That’s all the information we need,” he says. “That’s enough to tell us how much paint to buy.”

How did they go about it?

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The area of the deck is the area of the large circle minus the small circle. That’s πA² – πB², or π(A² – B²). We don’t know A or B, but we do know the length of the green line. That’s 42. So the Pythagorean theorem can tell us that B² + 21² = A², and hence that 21² = A² – B². Happily, we can plug this directly into the equation above and get our answer: The area of the deck is 21²π, or 441π square feet — regardless of the size of the carousel! Remarkably, this is true — so long as the green chord stays locked in position between them, the two circles can assume any size and the difference in their areas will remain constant. A similar principle holds in three dimensions. In The Universal Book of Mathematics, David Darling writes, “An even more amazing fact is that if you slide a chord of fixed length around any convex shape C so that the chord’s midpoint traces out another figure D, the area between C and D doesn’t depend on what shape you started with.” (Thanks, Dave.)

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August 8, 2012February 6, 2013 | Puzzles

Leap Day

Alice gets a rocket-powered pogo stick for her birthday. She jumps 1 foot on the first hop, 2 feet on the second, then 4, 8, and so on. This gets alarming. By judicious hopping, can she arrange to return to her starting point?

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No. After n hops, Alice will have traveled at most 1 + 2 + 4 + … + 2^n-1 = 2ⁿ – 1 feet from the starting point. But her very next hop will carry her 2ⁿ feet. So she must always overshoot the goal.

August 7, 2012February 6, 2013 | Puzzles

Desperate Measuring

desperate measuring

You’re alone on a desert island and want to lay out a course for some snail races. Unfortunately, you have only an 8.5 x 11 inch sheet of paper. How can you use it to measure exactly 3 inches?

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Here are two ways: “Dog-ear” the page, folding one corner across to the opposite side. The area below the dog-eared portion measures 8.5 x 2.5. Reopen the paper and fold this 8.5 x 2.5 portion up into the page. The area that remains uncovered measures 8.5 x 6; it can be folded in half to produce a height of 3 inches. Fold the paper in half to make a doubled sheet measuring 5.5 x 8.5. Now “dog-ear” this doubled page, folding one corner across to the opposite side. The area below the dog-eared portion measures 5.5 x 3.

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August 6, 2012February 6, 2013 | Puzzles

The Last Cent

You and a friend are playing a game. Between you is a pile of 15 pennies. You’ll take turns removing pennies from the pile — each of you, on his turn, can choose to remove 1, 2, or 3 pennies. The loser is the one who removes the last penny.

You go first. How should you play?

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Consider what happens if you leave your opponent 5 pennies. At that point, no matter how many he removes, you can play so as to leave the final penny to him. Now, if leaving him 5 pennies is a worthy target, then by the same principle so is leaving him 9 pennies: No matter how he plays, he must now give you the opportunity to leave him with 5. And going one step further, aiming for 13 pennies will ensure that you can reach 9, then 5, and victory. So, on your first turn, take 2 pennies.

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August 5, 2012February 6, 2013 | Puzzles

Fish Story

I release a fish at the edge of a circular pool. It swims 80 feet in a straight line and bumps into the wall. It turns 90 degrees, swims another 60 feet, and hits the wall again. How wide is the pool?

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It sounds as if there’s not enough information to solve this, but there is if you remember that when a right triangle is inscribed in a circle, its hypotenuse is the diameter. The pool is 100 feet wide.