Calculating Price Elasticity of Demand Hi I was given the following price vs quantity values.

Price   Quantity Demanded
4   221
5   210
6   185
7   162
8   144
9   122
10  102
11  81
12  61
13  46
14  25


The graph was plotted as shown below.



The equation was, $Y = -0.0496 X + 15.133$. What I need to know is, I was asked to find the PED when price is $\$7.5$.

Then what I did was, I found the quantity at price $7.5$ substituting to the price quantity equation. And then found the PED using the equation



The quantity derived for price $7.5$ was $153.89$. Then I calculated the PED as below. Is it correct?

$$\frac{(153.89-144)/144}{(7.5-8)/8} = 1.099$$

May I know whether this calculation is correct?
The answer will vary slightly depending on which notion of elasticity you're using. 

Arc elasticity (or midpoint elasticity) uses the formula
\begin{equation}
\epsilon^\text{arc}=\frac{Q_1-Q_0}{P_1-P_0}\cdot\frac{\frac12(Q_0+Q_1)}{\frac12(P_0+P_1)},
\end{equation}
where $\frac12(Q_0+Q_1)$ is the midpoint between $Q_0$ and $Q_1$. Note that in your case, $7.5$ is the midpoint between $7$ and $8$. 

Point elasticity uses the formula
\begin{equation}
\epsilon^\text{point}=\frac{\mathrm dQ}{\mathrm dP}\cdot\frac{P}{Q}.
\end{equation}
Here, $\frac{\mathrm dQ}{\mathrm dP}$ is the derivative of the demand function (evaluated at the point you want to calculate the elasticity, but it will be constant if demand is linear), and in your case, $P=7.5$ and $Q$ is the quantity demanded corresponding to that price.