Question 604594
How many 4-element subsets containing the letter A can be formed from the set {A,B,C,D,E,F,G}?
<pre>
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 {{{(matrix(2,1,6,3))}}}.  "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򈭽"

{{{(6*5*4)/(3*2*1)}}} = 20
 
Edwin</pre>