TITLE: Can elements in a set be duplicated?
QUESTION [15 upvotes]: If $A = \{x \mid x \text{ is a letter of the word 'contrast'}\}$
Represent it in a Venn Diagram, and then find the $n(A)$.

Do I need to write the letter 't' twice inside the venn diagram? What should be the answer for $n(A)$?

REPLY [18 votes]: From wikipedia:
Every element of a set must be unique; no two members may be identical. A multiset is a generalized concept of a set that relaxes this criterion.