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Topic: Hello I am looking for some help in some combinatorics/permutaions problems
Replies: 1   Last Post: Nov 15, 2011 11:59 PM

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 Federico Posts: 1 Registered: 11/15/11
Hello I am looking for some help in some combinatorics/permutaions problems
Posted: Nov 15, 2011 8:07 PM

1. Assuming a 7 digit telephone number is in the form of NXX-XXXX N = (2-9 and X = (0-9). How many numbers have exactly 3 zeroes?

I wrote 8 x C(9,2) but i got it wrong.

2. A club has 10 men and 9 women

Suppose two members of the club refuse to work together. How many groups of 5 can be chosen with this condition?

I wrote: C (19,5) + C(18,5) + C(18,5) but i got it wrong

3.In How many ways can 13 people be seated...

a)In a row, if 3 of the 13 must be seated next to one another?

b)In a circle, if 3 of the 13 must be seated next to one another?