Number of elements which belong to at most one of the sets. If $n(A) = n(B) = n(C) = 13$, $n(A\cap B \cap C') = 2$, $n(B \cap C \cap A') = 4$, $n(A \cap C \cap B') = 3$ and $n(A \cap B \cap C) = 1$, then find the number of elements which belong to at most one of the sets.
How to make Venn diagram in this question?
 A: Here is a Venn diagram that you can start with:

The different regions in the diagram, labeled as $I,II,III,\dots,VIII$ correspond to the following sets:
$\begin{array}{|l|l|}
\hline
\text{Region}&\text{Set}\\
\hline
I&A'\cap B'\cap C'\\
\hline
II&A\cap B'\cap C'\\
\hline
III&A\cap B\cap C'\\
\hline
IV&A'\cap B\cap C'\\
\hline
V&A\cap B'\cap C\\
\hline
VI&A\cap B\cap C\\
\hline
VII&A'\cap B\cap C\\
\hline
VIII&A'\cap B'\cap C\\
\hline
\end{array}$
Remember what these sets mean when interpreted in words.  $A\cap B\cap C'$ is the set of things in $A$ and in $B$ and not in $C$.  To be in this set on the Venn diagram, that corresponds to the region inside of the circle $A$, inside of the circle $B$ and outside of the circle $C$.  In other words, the region labeled in my picture as region $III$.
Now, looking at the problem statement, you are told $n(A\cap B\cap C'),n(A\cap B'\cap C), n(A'\cap B\cap C)$ and $n(A\cap B\cap C)$.  Go ahead and fill these in to the venn diagram.

From here, you want the total of all numbers in the circle for $A$ to be equal to $n(A)=13$.  What number should you place into region $II$?

 $13-2-1-3=7$

What about the other regions?
From all of this, you should be able to find out the number of elements in at least one of the regions as well as able to find out the number of elements in exactly one of the regions.  Do you in fact know anything about the universal set?
Without knowledge of the universal set, the question of at most one of the regions is unknown, since any number of elements might be in region $I$.
