Coefficient of $x^{41}$ in $(x^5 + x^6 + x^7 + x^8 + x^9)^5$ 
What is the coefficient of coefficient of $x^{41}$ in $(x^5 + x^6 + x^7 + x^8 + x^9)^5$?

Using summation of G.P., this is equivalent to finding the coefficient of $x^{41}$ in
$$\left(x^5 \left(\frac{1-x^5}{1-x}\right)\right)^5$$
and thus finding coefficient of $x^{16}$ in $(\frac{1-x^5}{1-x})^5$. How to proceed after this?

It's convenient to use the coefficient of operator $[x^k]$ to denote the coefficient of $x^k$ in a series. This way we can write e.g.
\begin{align*}
[x^k](1+x)^n=\binom{n}{k}
\end{align*}

We obtain
  \begin{align*}
[x^{16}]\left(\frac{1-x^5}{1-x}\right)^5
&=[x^{16}](1-x^5)^5\sum_{k=0}^\infty \binom{-5}{k}(-x)^k\tag{1}\\
&=[x^{16}](1-5x^5+10x^{10}-10x^{15})\sum_{k=0}^\infty \binom{k+4}{4}(-x)^k\tag{2}\\
&=\binom{20}{4}-5\binom{15}{4}+10\binom{10}{4}-10\binom{5}{4}\tag{3}\\
&=4845-5\cdot 1365+10\cdot 210-10\cdot 5\\
&=70
\end{align*}

Comment:


*

*In (1) we use the binomial series expansion of $\frac{1}{(1-x)^5}$

*In (2) it is sufficient to expand to binomial up to $x^{15}$ since we only need coefficients of powers of $x^{16}$. We also use the binomial identity
\begin{align*}
\binom{-n}{k}=\binom{n+k-1}{k}(-1)^k
\end{align*}

*In (3) we select the coefficients of the series accordingly

