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16 Aug 2014, 02:34
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Can any one tell me how to tell if the question ask about Permutation or Combination ? how to know when you read the question that the order matters or not.
i know that in Permutation the order matter and in Combination the order doesn't matter. and i know both formulas.
my problem is how to tell if the order matter or not ?
for ex :
Q-1 : Let’s say Dominic took 10 photos. He wants to put 7 of them on Facebook. How many groups of photos are possible?
Q-2 : how many possible options are there for the gold, silver, and bronze medals out of 12 athletes?
which question is for Combination and which one is for Permutation ? and how did you that ?
thanks
i know that in Permutation the order matter and in Combination the order doesn't matter. and i know both formulas.
my problem is how to tell if the order matter or not ?
for ex :
Q-1 : Let’s say Dominic took 10 photos. He wants to put 7 of them on Facebook. How many groups of photos are possible?
Q-2 : how many possible options are there for the gold, silver, and bronze medals out of 12 athletes?
which question is for Combination and which one is for Permutation ? and how did you that ?
thanks
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19 Aug 2014, 23:45
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Let's look at the two questions separately
Coming back to the question, if it were me, I would solve it as a combination question if no extra information is provided. Why I would do this is because all the photos are equivalent without additional information.
Whereas, if you were to rephrase the question to - 'Let’s say Dominic took 10 photos on 10 different days. He wants to put 7 of them on Facebook along with the date the photograph was taken. How many groups of photos are possible?'
In this case, the photos become unique if the date is important and I would gravitate towards permutations. (Note that this question is a bit vague as well and it is possible to draw different conclusions about what the question is asking. If this was an actual GMAT question, it would be clearer than I have made it.)
To conclude, solve any combinatorics problem using permutations or combinations. Then ask yourself if the order in the groups matters. Depending on the answer and the formula you have used, you can get the correct answer by dividing or multiplying.
Hope that makes it clear.
shagalo
We have 12 people and we want to choose 3 to give 3 distinct items. Here who receives what is going to matter and the question is asking the no of permutations possible.
shagalo
In this case the question is a bit unclear. If this was a GMAT question, in my opinion, it would be explicitly mentioned in the question whether the order matters or not. (Experts - I might be wrong about this, so please correct me if that is the case).
Coming back to the question, if it were me, I would solve it as a combination question if no extra information is provided. Why I would do this is because all the photos are equivalent without additional information.
Whereas, if you were to rephrase the question to - 'Let’s say Dominic took 10 photos on 10 different days. He wants to put 7 of them on Facebook along with the date the photograph was taken. How many groups of photos are possible?'
In this case, the photos become unique if the date is important and I would gravitate towards permutations. (Note that this question is a bit vague as well and it is possible to draw different conclusions about what the question is asking. If this was an actual GMAT question, it would be clearer than I have made it.)
To conclude, solve any combinatorics problem using permutations or combinations. Then ask yourself if the order in the groups matters. Depending on the answer and the formula you have used, you can get the correct answer by dividing or multiplying.
Hope that makes it clear.
20 Aug 2014, 21:51
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shagalo
It is a Combination problem when you have to 'Select'. Read it as - what am I required to do? Select 7 of the 10 photos to put up on Facebook. I need to find how many groups of photos are possible. After selection, I would have a bunch of 7 pictures. So this is a combination problem.
It is a Permutation problem when you have to 'Select and Arrange'. Read it as - what am I required to do? Select 3 of the 12 athletes for the medals and then arrange them in 1st, 2nd and 3rd spots (for Gold, Silver and Bronze)
or
Select one athlete for Gold (12C1), then one for Silver (11C1) and then one for Bronze (10C1) and then multiply them all to get 12*11*10. Both methods will lead to the same answer. This selection includes arrangement since we are selecting for each position separately.
In pictures on Facebook example, at the end of it all, we have a bunch of pictures. They are not arranged in any particular way. The same question could be made an arrangement question: he wants to select his 7 pictures and put them up on the walls of his 7 friends - now we don't have just a bunch of 7 pictures. We need to arrange them on 7 different walls. So this is Permutation now.
