Solution To Friday Puzzle #3

19 09 2011

John: “Hi, Mary, how’s it going?”

Mary: “Fine, thanks. Have you been thinking of that spinner problem someone blogged about on Friday?”

John: “Tell you what, I was just going to ask you the same. You wanna try solving it using the Monte Carlo method?”

Mary: “Monte who?

John: “Heehee…  Monte Carlo method is an experimental way to get approximate solutions to problems about probabilities. You repeat an experiment over and over and over, and make notes about the outcomes. When you have repeated the experiment a great many times you will have a pretty good idea of the probability you are trying to figure out. For instance, if you toss a coin ten times you may get 7 heads and 3 tails, but if you toss the same coin 10,000 times you may get 5,014 heads and 4,986 tails. The ratio will converge to 1/2 just like it should if the coin is good.”

Mary: “All right, let’s try it out. Hey, wait a minute, we don’t have any axles or blades to make spinners out of. How you gonna experiment with a device with… no device?”

John: “Mind you, I’ve thought of that, too! See, here are three pencils. Two of them are green and one is red. If the axles are called A and B, and the point at which the blades are pointing to is X, then the distance |AB| =1 must be greater than both of the distances |AX| and |BX| or else the blades can’t overlap.  In other words, the simulated blades will overlap if and only if the length |AB| is the longest side of the triangle ABX.”

Mary: “A-ha, I see now: I’ll toss the pencils on the floor, the pencils will define three straight lines, and those three lines together will define a random triangle.”

John: “Exactly! And next consider the red pen as the base between the axles and the green pens as the blades, then the experiment is successful every time when the red pen defines the longest side of the resultant random triangle. Count successes and divide by total numer of experimentations and voila, Monte Carlo method in action! Let’s start, toss the pencils, Mary.”

Mary: “But…”

John: “Yes I know we have a long task ahead of us so lets just start.”

Mary: “But…”

John: “Please, Mary, the longer we argue the longer it takes to find out the approximate solution. You do want to find it out, don’t you?”

Mary sighs and tosses the pencils.

John: “Well?”

Mary: “Well…”

John: “Well?”

Mary: “Well… ummm… errr… I am sorry. I was trying to tell you that I am color blind. I can’t tell apart red and green, but I can say that because there are three pencils, the red pencil defines the longest side of the triangle with probability 1/3.”

John is at loss of words in face of Mary’s spontaneous insight.

So the answer to Friday Puzzle #3 is that the blades overlap with probability P=1/3. Also, my apologies for failing to supply the solution by Sunday like promised.




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