NOTE: This is a mul

NOTE: This is a multi$-$part question. Once an answer is submitted, you will be unable to return to this part.

Consider strings of lower case letters of length $4$ or less.

Identify the formula to calculate the number of strings of lowercase letters of length four or less, not counting the empty string.

ti$-$part question. Once an answer is submitted, you will be unable to return to this part.

Consider strings of lower case letters of length $4$ or less.

Identify the rules used to calculate the number of strings of lowercase letters of length four or less, not counting the empty string. $($Check all that apply.$)$