Marginal probability derived from joint probability distribution of a poisson process and binomial process I thought I figured out this problem, but my answer is different than the solutions manual. Hoping someone here can stear me in the correct direction:

Assume the number of automobile collisions that occur on a given stretch of highway per year is a Poisson random variable $X$ with $\mu = 20$. The probability is $p=0.05$ that there will be one or more fatalities in each accident; occurrences of fatalities are independent from one collision to the next. If $Y$ is the number of collisions with one or more fatalities on this stretch of road in one year, find the probability law for $Y$.

So as said, X is poisson, so:
$$p_X(x) = \frac{20^x}{x!}e^{-20}$$
The conditional probability of $Y$ given $X=x$ is a Binomial random variable:
$$p_{Y\mid X}(y\mid x) = \binom{x}{y}p^yq^{x-y}$$
where $Y =$ number of fatal traffic accidents and $x =$ given number of accidents (from poisson distribution)
Therefore the joint probability for $X$ and $Y$ is:
$$p_{X,Y}(x,y)= p_{Y\mid X}\cdot p_X(x) = \binom{x}{y}p^yq^{x-y}\frac{20^x}{x!} e^{-20}$$
to find the marginal probability of $Y$, I sum over $P_{X,Y}$ for all $x$:
\begin{align}
p_Y(y) & = \sum_{x=y}^\infty{\binom{x}{y}p^yq^{x-y}\frac{20^x}{x!}e^{-20}} = \frac{(20p)^ye^{-20}}{y!} \sum_{x=y}^\infty {20^{x-y}\frac{q^{x-y}}{(x-y)!}} \\[10pt]
& = \frac{(20p)^ye^{-20}}{y!} \sum_{x-y=0}^\infty {20^{x-y}\frac{q^{x-y}}{(x-y)!}} = \frac{(20p)^ye^{-20}}{y!}e^{20q}
\end{align}
So: 
$$p_Y(y) = \frac{(20p)^ye^{-20p}}{y!}$$
however, the correct answer is apparently $p_Y(y)=\dfrac{e^{-1}}{y!}$

\begin{align}
& \sum_{x=y}^\infty \frac{20^x e^{-20}}{x!} \cdot \binom x y p^y (1-p)^{x-y} \\[12pt]
 = {} & \frac{e^{-20} p^y}{y!} \sum_{x=y}^\infty \frac{20^x(1-p)^{x-y}}{(x-y)!} \\[12pt]
= {} & \frac{e^{-20} p^y}{y!} \sum_{w=0}^\infty 20^{w+y} \frac{(1-p)^w}{w!} \\[12pt]
= {} & \frac{e^{-20} (20p)^y}{y!} \sum_{w=0}^\infty 20^w \frac{(1-p)^w}{w!} \\[12pt]
= {} & \frac{e^{-20} (20p)^y}{y!} \cdot e^{20(1-p)} \\[12pt]
= {} & \frac{e^{-20p} (20p)^y}{y!}.
\end{align}
So it's a Poisson distribution with expected value $20p$.
