Question

If $C$ is the set of letters in the word 'cooler' find $n\left(C\right)$

Answer 1

Set $C=\{c,o,l,e,r\}$
Elements in the set are unique, so we do not repeat the letters.

Thus, $o$ is written only once.
Now, $n\left(C\right)=5$ as there are $5$ elements in the set.

Hence, the answer is $5$.