circularsquare

Branch Unspecified

04 Apr 2012

**Simple Puzzle on unequal weights.**

You are given a weighing scale and 8 bags of rice. Seven bags are of equal weight. One is of a slightly heavier weight. You can compare the bags using the weighing scale. (note it doesn't give weight in kgs it gives just the comparison)

What is the minimum number of times you will have to perform a measurement on the weighing scale so that you can pinpoint the heavier bag.

Note :- 1) you can't use own hands to compare , the bag is heavier only slightly so only weighing scale can detect the difference.

2) weighing scale can hold an infinite weight on it.

Happy Puzzling. When you give answer , also show reasoning for benefit of others as is the custom of puzzles.😀

Many of you may have solved this one before in aptitude exams, it is common puzzle.

What is the minimum number of times you will have to perform a measurement on the weighing scale so that you can pinpoint the heavier bag.

Note :- 1) you can't use own hands to compare , the bag is heavier only slightly so only weighing scale can detect the difference.

2) weighing scale can hold an infinite weight on it.

Happy Puzzling. When you give answer , also show reasoning for benefit of others as is the custom of puzzles.😀

Many of you may have solved this one before in aptitude exams, it is common puzzle.

silverscorpion

Branch Unspecified

7 years ago

I can do it in 2 weighings.

Separate the bags into groups of three, and you will get three groups, 2 with 3 bags in it and 1 with 2 bags.

Now, weigh both the groups that have 3 bags. If they are of equal weight, then the odd one is in the other 2. If they are not equal, then whichever group is shown as heavier has the odd bag.

Now, if the heavier bag is in a group of 3, take any 2 bags from the group and compare. Depending on the result, you can tell which one is heavier.

If it is in the group of 2, weigh them and similarly, you can tell which is heavier.

So, the minimum number of weighings necessary would be two.

Separate the bags into groups of three, and you will get three groups, 2 with 3 bags in it and 1 with 2 bags.

Now, weigh both the groups that have 3 bags. If they are of equal weight, then the odd one is in the other 2. If they are not equal, then whichever group is shown as heavier has the odd bag.

Now, if the heavier bag is in a group of 3, take any 2 bags from the group and compare. Depending on the result, you can tell which one is heavier.

If it is in the group of 2, weigh them and similarly, you can tell which is heavier.

So, the minimum number of weighings necessary would be two.

circularsquare

Branch Unspecified

7 years ago

The answer is correct and the reasoning too. Well done.silverscorpionI can do it in 2 weighings.

Separate the bags into groups of three, and you will get three groups, 2 with 3 bags in it and 1 with 2 bags.

Now, weigh both the groups that have 3 bags. If they are of equal weight, then the odd one is in the other 2. If they are not equal, then whichever group is shown as heavier has the odd bag.

Now, if the heavier bag is in a group of 3, take any 2 bags from the group and compare. Depending on the result, you can tell which one is heavier.

If it is in the group of 2, weigh them and similarly, you can tell which is heavier.

So, the minimum number of weighings necessary would be two.

circularsquare

Branch Unspecified

7 years ago

Perhaps you didn't take into consideration the fact that the scale can handle infinite weight. Hence one could keep 3 bags on each side at once :-BionicWhat was I thinking? Oh my God...

circularsquare2) weighing scale can hold an infinite weight on it.

Yes Yes Yes Yes.....circularsquarePerhaps you didn't take into consideration the fact that the scale can handle infinite weight. Hence one could keep 3 bags on each side at once :-

I didn't even consider the inifinite no. Of weight the weighing scale can hold.

I was thinking its the fact that it can only hold two weights at once.

CE Designer

Branch Unspecified

7 years ago

What if we were using a digital scale? What would be the answer then?😨

Kaustubh Katdare

Electrical

7 years ago

That'd be another interesting puzzle! 😀CE DesignerWhat if we were using a digital scale? What would be the answer then?😨

in that case precise answer may take number of iterations 😀 other wise i would go with 3 iterations beginning with 4 bags on each weigh ,the heavier one will be divided again into 2 weigh each and then finally i can come to resultsCE DesignerWhat if we were using a digital scale? What would be the answer then?😨

CE Designer

Branch Unspecified

7 years ago

I have a solution with 4 iterations too, can anyone do it with less?

Now it gets a little tricky, from the first time we weighed one set of four was heavier than the other. The lighter set, lets say it weighed 10kg. This is what the heavier set

So two bags from this set would weigh 5kg.

Now back to step 3. We have 2 sets of two. We know one pair will weigh 5 and the other lets say 11.

4.Weigh again. Thats 3 times.

5.You will either get 5 kg or 6 kg. If you get 5kg you know the other pair has the heavier sack.

6.Weigh one from that pair to determine which is heavier. Thats 4.

7.If you get 6kg the first time just pull one off the scale and you will know which is heavier. Thats also 4.

I hope i explained well, I also hope someone gets it in less iterations (SS) 😎

- I start with splitting them into 4 bags like vinci.
- Weigh both sets. Heavier set is what we want. Thats 2.
- Divide the heavier set in two (2 x 2).

Now it gets a little tricky, from the first time we weighed one set of four was heavier than the other. The lighter set, lets say it weighed 10kg. This is what the heavier set

*should*weigh.So two bags from this set would weigh 5kg.

Now back to step 3. We have 2 sets of two. We know one pair will weigh 5 and the other lets say 11.

4.Weigh again. Thats 3 times.

5.You will either get 5 kg or 6 kg. If you get 5kg you know the other pair has the heavier sack.

6.Weigh one from that pair to determine which is heavier. Thats 4.

7.If you get 6kg the first time just pull one off the scale and you will know which is heavier. Thats also 4.

I hope i explained well, I also hope someone gets it in less iterations (SS) 😎

CE Designer

Branch Unspecified

7 years ago

You would have to weigh the two sets of 4 to determine which is heaviervincii would go with 3 iterations beginning with 4 bags on each weigh ,the heavier one will be divided

silverscorpion

Branch Unspecified

7 years ago

I tried.. I couldn't do it in 3 weighings.. So , I think 4 is the least number of weighings we can do it, using a digital balance.CE DesignerI hope i explained well, I also hope someone gets it in less iterations (SS) 😎

By the way, it will be significantly more difficult if it's not known that one of the bags is "heavier" for sure.

That is, if it is only known that one of the bags is of a different weight than the others, and it's not specified whether it's heavier or lighter, it'll be more difficult. I think The minimum number of iterations required to identify the odd one, will also be more. Just try 😀

not true, try again ok?silverscorpionI tried.. I couldn't do it in 3 weighings.. So , I think 4 is the least number of weighings we can do it, using a digital balance.

By the way, it will be significantly more difficult if it's not known that one of the bags is "heavier" for sure.

That is, if it is only known that one of the bags is of a different weight than the others, and it's not specified whether it's heavier or lighter, it'll be more difficult. I think The minimum number of iterations required to identify the odd one, will also be more. Just try 😀

silverscorpion

Branch Unspecified

7 years ago

^^ You mean we can do it in less than three trials??

In case of digital machine, this will be different but still minimum is 2 😉

Make same sets as for analog - 2 of 3 bags and 1 of 2 bags

Set 1: 3 bags (Bag1,Bag2,Bag3)

Set2: 3 bags (Bag4,Bag5,Bag6)

set 3: 2 bags (Bag7,Bag8)

If value of 1st iteration > val of 2nd ietration => Bag7 is heavier else Bag8 is heavier

If value of 1st iteration == val of 2nd iteration

3rd iteration Put entire Set1 on machine

case 1 : If its value is exactly 3 times of Bag7/bag8 then

Add 1 bag Bag4 from Set2 if its value is exactly 4 times of Bag7/Bag8 or not if not then Bag4 is heavier else put another Bag5 from Set2 and check if its value is exactly 5 times of Bag7/Bag8 if not then Bag5 is heavier else remaining Bag6 is heavier

case 2: If its value is more than 3 times of Bag7/bag8 then

Remove one Bag3 from Machine if its weigh is exactly 2 times of Bag7/Bag8 then Bag3 is heavier else remove another Bag2 from Machine if its weigh is exactly equal to Bag7/Bag8 then Bag2 is heavier else Bag1 is heavier.

Is this the approach ?

Make same sets as for analog - 2 of 3 bags and 1 of 2 bags

Set 1: 3 bags (Bag1,Bag2,Bag3)

Set2: 3 bags (Bag4,Bag5,Bag6)

set 3: 2 bags (Bag7,Bag8)

**Step1 :**1st iteration weigh Bag7 out of Set3. Note down its value.**Step 2:**2nd iteration weigh Bag8 out of Set3. Note down its value.If value of 1st iteration > val of 2nd ietration => Bag7 is heavier else Bag8 is heavier

If value of 1st iteration == val of 2nd iteration

**Step3:**Now take advantage of digital machine over analog3rd iteration Put entire Set1 on machine

case 1 : If its value is exactly 3 times of Bag7/bag8 then

Add 1 bag Bag4 from Set2 if its value is exactly 4 times of Bag7/Bag8 or not if not then Bag4 is heavier else put another Bag5 from Set2 and check if its value is exactly 5 times of Bag7/Bag8 if not then Bag5 is heavier else remaining Bag6 is heavier

case 2: If its value is more than 3 times of Bag7/bag8 then

Remove one Bag3 from Machine if its weigh is exactly 2 times of Bag7/Bag8 then Bag3 is heavier else remove another Bag2 from Machine if its weigh is exactly equal to Bag7/Bag8 then Bag2 is heavier else Bag1 is heavier.

Is this the approach ?

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