26)

matter, so we count the number of combinations: 9C4=9!/(5!

*4!)=126

different, that is why we will calculate the number of

permutations (when order matters): 7P5=7!/2!=2520

of combinations:

nPk÷nCk = n!/(n-k)!÷n!/[(n-k)!k!]= n!/(n-k)!× [(n-k)!k!]/n!=k!