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FrankLu99
10-26-2004, 07:19 PM
How do i quickly solve by hand the following type of math problem

Calculate the number of possible combos when out of X items I choose Y items in any order.

Ex
10 items, I choose 4.

thanks
yes i slept thru stats/probablity class (9am class)

hotquietday
10-26-2004, 07:33 PM
In general, the formula for nCr -- the number of ways to choose r items from a total of n items -- is given by this forumla:

n!
------
r! (n-r)!

where n! = n*(n-1)*(n-2)* ... *3*2*1

Thus in your example 10C4 we have (after cancelling similar terms):

10*9*8*7*6*5
------------ = 1260
4*3*2

Cheers,
John

FrankLu99
10-26-2004, 07:36 PM
[ QUOTE ]
In general, the formula for nCr -- the number of ways to choose r items from a total of n items -- is given by this forumla:

n!
------
r! (n-r)!

where n! = n*(n-1)*(n-2)* ... *3*2*1

Thus in your example 10C4 we have (after cancelling similar terms):

10*9*8*7*6*5
------------ = 1260
4*3*2

Cheers,
John

[/ QUOTE ]

ah crap. i had a bad feeling it was the formula with factorials.

thanks

hotquietday
10-26-2004, 07:42 PM
[ QUOTE ]
ah crap. i had a bad feeling it was the formula with factorials.

[/ QUOTE ]

Don't be scared of factorials. You don't need to do them by hand. There are tons of free downloadable calculators that will do them for you.

One called TextCalc you can just type nCr(10,4) for example to do the calcualtion above.

J

FrankLu99
10-26-2004, 07:50 PM
Thanks again but I already have a calculator that does it. I just wanted to understand how it arrives at its answer.

BruceZ
10-27-2004, 04:05 AM
[ QUOTE ]
In general, the formula for nCr -- the number of ways to choose r items from a total of n items -- is given by this forumla:

n!
------
r! (n-r)!

where n! = n*(n-1)*(n-2)* ... *3*2*1

Thus in your example 10C4 we have (after cancelling similar terms):

10*9*8*7*6*5
------------ = 1260
4*3*2

Cheers,
John

[/ QUOTE ]

That's incorrect. 10C4 = 10*9*8*7 / (4*3*2*1) = 210.

hotquietday
10-27-2004, 04:43 AM
[ QUOTE ]
That's incorrect. 10C4 = 10*9*8*7 / (4*3*2*1) = 210.

[/ QUOTE ]

Well, the formula is correct. I just applied it wrong for some reason.

Thanks for that though,
John