Real MATHEMATICS – Game #8

Judge Seesaw 2.0.1

In the previous post I asked how you can find the 12th box when we don’t know whether it is heavier or lighter than the rest of the boxes.

To start the solution, we will divide the boxes into three groups of 4. But this time I will assign names to these groups and boxes:

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Eleven of them are identical. Which one is the oddly-weighted one?

In the first try we’ll place the groups A and B to the end of the seesaw. There will be two possible outcomes:

Outcome 1: Seesaw is in balance.

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This outcome shows that the boxes of A and B are identical.

Then, we can conclude that oddly-weighted box is among the group C.

In the second try let’s place any three boxes of the group C with any three boxes of the group A. (Why three from A? Since all the boxes of A and B are identical, it doesn’t matter which three boxes I choose among them. I selected three from the group A. I could have selected two boxes from A and one from B as well. It wouldn’t change anything.)

After the second try, we will again left with two possible outcomes:

a. Seesaw is in balance.

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C4 is the oddly-weighted one.

It means three boxes of C and A are identical. Then we can conclude that the forth box of C is the oddly-weighted box.

b. Seesaw is tilted.

This concludes that one of the three boxes I choose from C is the oddly-weighted box. In this case the seesaw is either tilted towards C or A which tells us our box is either heavier or lighter than the rest of the boxes.

In other words, after the second try we ended up with the following question: “Two of three boxes have the same weight. If you know whether the odd box is heavier or lighter than the other two, how can you find the odd box?”

This can be solved with exactly one try on the seesaw. Select any two of the three boxes, place them on the seesaw.

If seesaw is in balance, then the third box is the odd one:

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C2 is the oddly-weighted one.

In case seesaw is tilted, as we know whether the odd box is heavier or lighter than the others, we can find the box we’ve been looking for:

Outcome 2: Seesaw is tilted.

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In this case the box we are looking for is either inside the group A or inside the group B.

Another deduction we can make is that one of these groups is heavier than the other. For the sake of a clearer solution I am going to assume that A is heavier than B. (I could have chosen the other way around; it wouldn’t change anything.)

Before starting the second try I will be swapping a box between A and B. Also I will be replacing the remaining three boxes of B with three boxes of C.

There are three possible outcomes:

a. Seesaw is balanced.

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Our box is either one of B2, B3 or B4

This means that all the boxes sitting on the seesaw are identical. Hence the box we are looking is among the three boxes of group B which we replaced. We can also deduce that oddly-weighted box is lighter than the other boxes since the seesaw is balanced.

At this point all we need to do is to find the lighter one inside three boxes. As we showed in the previous situation, it can be done with comparing any two of those boxes.

b. Seesaw is tilted in the same direction as the first try.

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Our box is either one of A1, A2 or A3.

This means that the oddly-weighted box is among the three boxes of A. We can also conclude that this box is heavier than the rest.

Again we have three boxes with one of them heavier than the other two. We can find that box with simply comparing any two of those boxes with the seesaw.

c. Seesaw is tilted in the opposite direction of the first try.

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Our box is either A4 or B1.

This means that the oddly-weighted box is either one of the boxes which we swapped between A and B. But in this situation we have no clue if the box we are looking for is heavier or lighter.

In order to find our box, we should compare one of those two boxes with any one of the remaining 10 boxes.

If there is a balance on the seesaw, it means our box is the one we didn’t place on the seesaw:

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Our box is A4.

If the seesaw is tilted, then it means our box (in the following cases it is A4) is the one we’ve chosen and placed on the seesaw.

M. Serkan Kalaycıoğlu

Real MATHEMATICS – Game #7

Judge Seesaw

There are 12 gift boxes on the table. They all have identical sizes and gift wraps.

rectangular white and red gift box

11 of them weight the same as the 12th box weights different from others.

Goal: Finding the oddly-weighted box.

Prize: 50 million old Turkish liras worth Oreo. (Because Oreo is the most beautiful thing in life.)

Tools: You can use a seesaw to compare the weights of the boxes.

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Challenge: You will have to guess the oddly-weighted box in at most 3 tries.

Judge Seesaw 1.0.1

It is the most basic version of the game where it is known if the 12th box is heavier or lighter than other 11 boxes.

At this point give yourself a few minutes and think on a solution when it is known that the 12th box is heavier than the rest.

***

Time is up. Here is the solution:

I assumed that the 12th box is heavier than the rest of the boxes. Then we should divide 12 boxes into three groups of 4.

In the first try we will take any two of those groups of 4 and place them on the ends of the seesaw. There will be exactly two possible outcomes:

Situation A: Seesaw is balanced.

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Two groups have equal weights which would mean the oddly-weighted box must be among the third group of 4.

 

In the second try we will take the third group of 4, divide those two-by-two and place them on the ends of the seesaw.

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We will be seeing that the seesaw is tilted to one side. That happens because heavy box sits on the seesaw.

 

In the third try we’ll be taking the heavier couple from previous measure and place them one-by-one on the ends of the seesaw.

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The seesaw will be tilted in favor of the heavier box. Hence we found the oddly-weighted box in exactly three tries.

 

Situation B: Seesaw is tilted.

tahte2

 

This happens because oddly-weighted box is among one of the groups of 4 boxes. Take that group and apply the same methods we did in situation A.

One wonders…

Judge Seesaw 2.0.1

In this version of the game we have no idea whether the 12th box is heavier or lighter than the rest of the boxes.

Even though it seems like a slight change, this game has become much harder compared to the version 1.0.1.

This is why I will give you a little time. I’ll include the answer in the next post.

M. Serkan Kalaycıoğlu