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19 Jul 2017, 08:19
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I have gone through the GMAT club Math book, and I know it states that we need to first figure out what the questions asks rather than ponder on which formula to use, but I still get confused about which formula to use, esp for the 'selecting a committee' / 'selecting x from y' etc.
I also get confused scenarios where to use the aCn formula and the (I don't know what it is called) n x (n-1) x (n-2) x (n-3) formula
example:
Q. From a group of 8 members, 4 are to be selected to form a committee.
- My thought process:
I get confused whether to use 8C4 or do 8x7x6x5
Is there an easy method to identify when to use each of these formulas:
nCa
nPa
n!
n x (n-1) x (n-2)...
I also get confused scenarios where to use the aCn formula and the (I don't know what it is called) n x (n-1) x (n-2) x (n-3) formula
example:
Q. From a group of 8 members, 4 are to be selected to form a committee.
- My thought process:
I get confused whether to use 8C4 or do 8x7x6x5
Is there an easy method to identify when to use each of these formulas:
nCa
nPa
n!
n x (n-1) x (n-2)...
19 Jul 2017, 09:21
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ameyaprabhu
I have gone through the GMAT club Math book, and I know it states that we need to first figure out what the questions asks rather than ponder on which formula to use, but I still get confused about which formula to use, esp for the 'selecting a committee' / 'selecting x from y' etc.
I also get confused scenarios where to use the aCn formula and the (I don't know what it is called) n x (n-1) x (n-2) x (n-3) formula
example:
Q. From a group of 8 members, 4 are to be selected to form a committee.
- My thought process:
I get confused whether to use 8C4 or do 8x7x6x5
Is there an easy method to identify when to use each of these formulas:
nCa
nPa
n!
n x (n-1) x (n-2)...
I also get confused scenarios where to use the aCn formula and the (I don't know what it is called) n x (n-1) x (n-2) x (n-3) formula
example:
Q. From a group of 8 members, 4 are to be selected to form a committee.
- My thought process:
I get confused whether to use 8C4 or do 8x7x6x5
Is there an easy method to identify when to use each of these formulas:
nCa
nPa
n!
n x (n-1) x (n-2)...
Hi...
1) when you are selecting say 3 things out of n things, it is nC3. Here a,b,c is same as b, c,a..
2) when you are arranging say 3 things out of n things, it is nP3. Here a, b, c is different from b, a, c..
It is also same as n*(n-1)*(n-2)....
3) n! Is when you are arranging all n of n things... Same as n*(n-1)*(n-2)*.....3*2*1
19 Jul 2017, 20:52
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Hi chetan2u,
I am still a little confused here.
You have mentioned we need to use nPa for arrangements, but the GMAT club Math book says we need to use this formula for "choosing", albeit for an ordered collection.
How do we differentiate between ordered and unordered?
I am still a little confused here.
You have mentioned we need to use nPa for arrangements, but the GMAT club Math book says we need to use this formula for "choosing", albeit for an ordered collection.
How do we differentiate between ordered and unordered?
)
