Use the union rule to answer the question.
If $n(B)=13,n(A\cap B)=4$ and $n(A\cup B)=33$, what is $n(A)?$

Step 1 ：Given that the number of elements in set B, denoted as $n(B)$, is 13, the number of elements in the intersection of sets A and B, denoted as $n(A\cap B)$, is 4, and the number of elements in the union of sets A and B, denoted as $n(A\cup B)$, is 33.

Step 2 ：We are asked to find the number of elements in set A, denoted as $n(A)$.

Step 3 ：We can use the union rule, which states that $n(A\cup B)=n(A)+n(B)-n(A\cap B)$.

Step 4 ：Rearranging this formula to solve for $n(A)$, we get $n(A)=n(A\cup B)-n(B)+n(A\cap B)$.

Step 5 ：Substituting the given values into this formula, we get $n(A)=33-13+4$.

Step 6 ：Simplifying this, we find that $n(A)=24$.

Step 7 ：So, the number of elements in set A is $\boxed{{\displaystyle 24}}$.