The [ratio](https://artofproblemsolving.com/wiki/index.php/Ratio) of boys to girls in Mr. Brown's math class is $2:3$.  If there are $30$ students in the class, how many more girls than boys are in the class?
$\text{(A)}\ 1 \qquad \text{(B)}\ 3 \qquad \text{(C)}\ 5 \qquad \text{(D)}\ 6 \qquad \text{(E)}\ 10$
Let the number of boys be $2x$.  It follows that the number of girls is $3x$. These two values add up to $30$ students, so \[2x+3x=5x=30\Rightarrow x=6\]
The [difference](https://artofproblemsolving.com/wiki/index.php/Subtraction) between the number of girls and the number of boys is $3x-2x=x$, which is $6$, so the answer is $\boxed{\text{D}}$.