TeX source:

\begin{array}{l} \% \text { change in quantity } &=\frac{Q_{2}-Q_{1}}{\left(Q_{2}+Q_{1}\right) / 2} \times 100 \\ \% \text { change in price } &=\frac{P_{2}-P_{1}}{\left(P_{2}+P_{1}\right) / 2} \times 100 \end{array}