Question 255054
How many different four-digit numbers can be formed using the digits 1, 1, 9, and 9?
1199
1919
1991
9119
9191
9911
combination formula, or the number of ways to combine k items from a set of n:
{{{C(n,k) =n!/((n-k)!k!)}}}
there are 4 numbers so n=4
there are only 2 different numbers we are actually choosing so k=2
{{{C(n,k) =4!/((4-2)!2!)}}}
{{{C(n,k) =(4*3*2*1)/(2!2!)}}}
{{{C(n,k) =24/(2*1*2*1)}}}
{{{C(n,k) =24/4}}}
{{{C(n,k) =6}}}