Suppose we define ℓ(n) as follows: If n is an integer from 0 to 20, inclusive, then ℓ(n) is the number of letters in the English spelling of the number n; otherwise, ℓ(n) is undefined. For example, ℓ(11)=6 because "eleven" has six letters, but ℓ(23) is undefined, because 23 is not an integer from 0 to 20
How many numbers are in the domain of ℓ(n) but not in the range of ℓ(n)?
Suppose we define $\ell(n)$ as follows: If $n$ is an integer from $0$ to $20,$ inclusive, then $\ell(n)$ is the number of letters in the English spelling of the number $n;$ otherwise, $\ell(n)$ is undefined. For example, $\ell(11)=6,$ because "eleven" has six letters, but $\ell(23)$ is undefined, because $23$ is not an integer from $0$ to $20.$
How many numbers are in the domain of $\ell(n)$ but not the range of $\ell(n)?$