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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 NXXXXXX N = (29 and X = (09). 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?
Thanks in advance !



