favorite number

What’s The 21st Prime Number? 73 is the 21st prime number. Its mirror (37) is the 12th and its mirror (21) is the product of multiplying 7 and 3. In binary, 73 is a palindrome, 1001001 which backwards is 1001001.” Why is 73 Special? “The best number is 73,” Cooper explained in the episode. “Why?