Find the sum of the arithmetic series
\[20+20\frac{1}{5}+20\frac{2}{5}+\cdots+40\]
$\textbf{(A)}\ 3000 \qquad\textbf{(B)}\ 3030 \qquad\textbf{(C)}\ 3150 \qquad\textbf{(D)}\ 4100 \qquad\textbf{(E)}\ 6000$
Brief Introduction
For those that do not know the formula, the sum of an arithmetic series with first term $a_1$, last term $a_n$ as $n$ terms, is \[S = \frac{n(a_1+a_n)}{2}.\] We can prove this as follows:
Let $d$ be the common difference between terms of our series and let $n$ be the number of terms in our series. Let $a_1$ be the first term. Our series is \[a_1,~a_1+d,~a_1+2d,\dots,a_1+(n-1)d.\] Note that we have $n-1$ in the last term because $a_1$ is a term. Let $S$ be our sum such that \[S=a_1+(a_1+d)+\dots+(a_1+(n-1)d).\] We can rewrite our sums as \[S=(a_1+(n-1)d)+(a_1+(n-2)d)+\dots+(a_1+d)+a_1.\] Adding these two sums together essentially creates $n$ pairs of $a_1+(a_1+(n-1)d)$ as shown below: \[2S=n(a_1+(a_1+(n-1)d))\implies S=\frac{n(a_1+a_n)}{2}\] We use $a_n$ in place of $a_1+(n-1)d$ to represent the last term.



Solving
Our first term is $20$ and our last term is $40$. To find the number of terms, $n$, we note that the common difference between each term is $\frac{1}{5}$. So we have \[20+\frac{1}{5}(n-1)=40\implies n-1=100\implies n=101.\] Using our formula, our sum is \[101\left(\frac{20+40}{2}\right)=101\times 30=\boxed{\textbf{(B) }3030.}\]

--Solution by [TheMaskedMagician](https://artofproblemsolving.comhttp://www.artofproblemsolving.com/Forum/memberlist.php?mode=viewprofile&u=200685)