Consider the set of all possible two - digit numbers that can be created using an unlimited supply of 1s, 3s, 7s and 9s. What is the greatest absolute difference between any two primes in this set?

-Consider the set of all possible two-digit numbers that can be created using an unlimited supply of 1s, 3s, 7s and 9s. What is the greatest absolute difference between any two primes in this set?
-In how many different ways can the letters of CHAIRS be arranged?