The Best Math or Logic Puzzle Ever
Here is a logic puzzle that this author would bet you won't solve quickly. It is not for the faint of heart or those who give up quickly. While the answer is given below, I ask that you try—and try hard—to solve the riddle on your own before examining the answer. While the puzzle is based on both math and logic, it is primarily a logic riddle and should be treated as such.
You begin with 12 coins and the knowledge that one of these coins is counterfeit. The coins look identical, and the only difference between them all is that the counterfeit coin is of a different weight. You don't know whether it is lighter or heavier than the rest of the coins, just that it will weigh differently.
Your only tool is a set of old fashioned scales; the type with two pans hanging from a balancing arm (see photo below). When more weight is placed on one pan than the other, that pan will sink while the lighter one will rise.
You have just three weighings to find the counterfeit coin. As you discover which one is counterfeit, you should also be able to tell if it is heavier than the rest or lighter. Good luck!
Your objective, of course, is to find the solution before reading how to solve the riddle. When you give up a week from now, come back and scroll slowly down to the next section; the first few sentences will give a valuable hint. Of course, if you figure it out come back and let us all know in the poll at the bottom that it happened, or leave your online name in the comment section.
Finding the Solution
Let's start by numbering the coins from 1 to 12, just so that they can be identified in this text. The actual solution doesn't require this, but it certainly makes it easier to describe the actions to be taken in solving the puzzle.
Begin by weighing coins 1, 2, 3, and 4 against 5, 6, 7, and 8.
STOP HERE if you are looking for a hint. That's it—weigh four coins against four coins. If you have given up and are looking for the answer, continue reading.
There are three possibilities. Let's call them case 1, case 2 and case 3.
- Case 1 is if both sides weigh the same; all the coins from 1 to 8 have the same weight, and none are counterfeit. Scroll down for the second weighing in case 1.
- Case 2 is if the left side, with coins 1, 2, 3, and 4 is heavier and causes that side sinks. This means that either one of these coins is heavy (and counterfeit) or that one of the coins 5, 6, 7, or 8 is lighter and thus fake. Scroll down to see the second weighing for case 2.
- Case 3 is the mirror image of case 2; the right side sinks. The solution for case 3 is analogous to case 2 and is left to the reader. The steps will be the same; you must simply visualize a different weight.
Case 1: During Weighing 1, Both Sides Were the Same
The obvious conclusion is that one of coins 9, 10, 11, or 12 is counterfeit. For your second weighing, put coins 9, 10, and 11 on the left side and 1, 2, and 3 on the right side. There are, once again, three possibilities; case 4 where both sides are equal, case 5 where the left side goes down because it is heavier, and case 6 where the left side goes up because it is light. Bear in mind here that the right side contains only legitimate coins; if the left side goes down it isn't because a light coin is on the right side, it is because a heavy coin is on the left.
In case 4, the solution is obvious; the bad coin is #12; the only one not proven to be of equal weight with all the others. For a third weighing, that coin can be measured against any other coin to determine if it is light or heavy.
For case 5, scroll down to find the third weighing if you haven't figured it out already.
Case 6 is again analogous to case 5 and is left to the reader to find.
Case 2: During Weighing 1, the Left Side Is Heavier
This is the trickiest of all the possibilities. The author assumes you haven't figured it out—if you had you wouldn't be here! Let's look at it.
For the second weighing, put coins 1, 2, 3, and 5 on the left pan and 9, 10, 11 and 4 on the right side. Remember that we already know that 9, 10, and 11 are all good coins.
There are (as always) three possibilities. Case 7 is that both sides weigh the same, Case 8 is that the left side goes down. Case 9 is the right side goes down.
Case 7 tells us that the bad coin is number 6, 7, or 8 and that it is lighter than all the rest (remember weighing #1 where the side with those coins went up). All other coins have been proven to be of equal weight. For the third weighing, weight 6 against 7; the one that goes up is the bad coin while if they are equal the final answer is coin 8.
Case 8 tells us that the bad coin is #1, 2 or 3 and that it is heavy. Were the problem in either coin 4 or 5 the scales would go the other way. For the third weighing test coin 1 against coin 2. If one of them is heavy it is bad while if they are equal in weight the bad coin is number 3.
Case 9 is that the right side of weighing 2 goes down. This can only happen if coin 4 is heavy or coin 5 is light. For the third weighing test coin 4 against coin 1 (a known good coin). If unequal coin 4 is bad and heavy, if equal coin 5 is bad and light.
Case 5: During Weighing 2, the Left Side Is Heavier
The conclusion to date is that the bad coin is number 9, 10, or 11 and that it is heavy. That side of the scales went down; it must be heavy.
As has already been seen, the solution to this scenario is to weigh #9 against #10; if one is heaver that is the bad coin, while if they are equal the bad coin is #11.
This finishes the solution to one of the very best math and logic puzzles or riddles. For those readers interested in mathematical problems, you might find Zeno's Paradox of interest. This was found in about 400 BC and has baffled thinkers ever since.
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© 2011 Dan Harmon