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Coin on a Chessboard

Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.


Puzzle ID:#5328
Fun:*** (2.4)
Difficulty:*** (2.62)
Submitted By:cathalmccabe***
Corrected By:cnmne




If a chessboard is the size of 80cm, and you throw a coin onto it of diameter 2cm. The centre of the coin falls somewhere within the 64 squares. What is the probability that it lands fully on a white square?

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Jul 18, 2002

Good one... once I got over the thought that it may have been a riddle - heh, good thing these categories. Once I knew it was mathematical, it fell into place.
Oct 23, 2002

Not to be too picky, but that would be true only if the puzzle stated, "The ENTIRE coin falls within one of the 64 squares." The puzzle currently says the "CENTER" of the coin will land within one of the 64 squares. This means that the coin can land anywhere on the board except for the center of it landing exactly on a line. Cool teaser though - sorry I had to get critical.
Nov 08, 2002

Incorrect, sack. If the question stated that the entire coin landed in one of the 64 squares, the answer would obviously be 1/2, since half the squares are white. It is important in this teaser that the coin's center can land anywhere on the board with equal probability.
Dec 28, 2002

this will sound wrong and dirrty but i agree with bender! + isn't there a simpler explanation!
Mar 14, 2003

Another good one from cath. Well thought out and the conditions were clear and precise. I liked it!
Aug 15, 2007

The question seemed to be a bit ambiguous, making it uncertain if the board was 80 centimeters squared or 80 centimeters on a side. If it is 80 centimeters on a side, each square would be 10 cm (about 4 inches) square, rather a huge chessboard, I would think.
Interesting teaser though.
Jul 09, 2009

That was fun, did it in about 60 seconds, I posed a similar problem a long time ago on here with a square of side 100 cm, fully containing 2 smaller squares each of side 17 cm. The question was to find the probablity they touch or overlap, I had a nice solution using triangles
Oct 15, 2010

My eyes are bleeding from seeing 32/100 instead of 4/25!!!
Oct 15, 2010

I mean 8/25 X_X

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