Give your answer using permutation notation, factorial notation, or other operations. Then evaluate. How many permutations are there of the letters in each of the following words, if all the letters are used without repetition?
Social Security Numbers. $\quad$ A social security number is a 9 -digit number like $243-47-0825$
a) How many different social security numbers can there be?
b) There are about 310 million people in the United States. Can each person have a unique social security number?