If $A$ and $B$ are two sets such that number of elements in $A$ is $20$, number of elements in $B$ is $18$ and number of elements in both $A$ and $B$ is $10$, find $n(A\cup B)$.

According to the fundamental formula of set theory:
$$n(A\cup B)=n(A)+n(B)-n(A\cap B)$$

Given values:

* Number of elements in set $A$, $n(A)=20$
* Number of elements in set $B$, $n(B)=18$
* Number of elements in both $A$ and $B$, $n(A\cap B)=10$

Substituting the values into the formula:

$$n(A\cup B)=20+18-10$$

$$n(A\cup B)=38-10=28$$

Therefore, the correct option is D.