I'm reading the problem from this stanford material (http://infolab.stanford.edu/~ullman/focs/ch04.pdf). Can you please help me understand this?
Question: At Real Security, Inc., computer passwords are required to have four digits (of $10$ possible) and six letters (of $52$ possible). Letters and digits may repeat. How many different possible passwords are there?
** my approach** So for the $10$ password chars: $10^4$ (for digits) and $52^6$ (for letters)
But how do I combine these permuting groups?