Question 604594: How many 4-element subsets containing the letter A can be formed from the set {A,B,C,D,E,F,G}?
Answer by Edwin McCravy(20055)   (Show Source):
You can put this solution on YOUR website! How many 4-element subsets containing the letter A can be formed from the set {A,B,C,D,E,F,G}?
You just have to choose the 3-element subsets taken from
the other 6 members
{B,C,D,E,F,G}
to put with the A to make a 4-element subset
containing the letter A.
That's "6 things Choose 3", written sometimes as 6C3, sometimes
as C(6,3), and sometimes as  . "The number of combinations of
6 things taken 3 at a time". The formula is
"Start with 6, write 3 factors 6 and divide by 3! or 3"
 = 20
Edwin
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