The demand for a certain item is $$\displaystyle{Q(p) = -0.8p+15}.$$

$p$ represents the price of an item and $Q$ represents the number of items sold at that price.

Evaluate the demand elasticity $E$ when $p=10$:

$E(10)$ =

Here, "demand elasticity" is the absolute value of $\displaystyle \frac{\text{percent change in quantity}}{\text{percent change in price}}$ for an infinitesimal change in price, so $$E = \left| \lim_{\Delta p \to 0} \frac{ \Delta Q / Q}{\Delta p / p} \right| \ .$$