TeX source:

\begin{aligned} \% \text { change in quantity } &=\frac{1,600-1,800}{(1,600+1,800) / 2} \times 100 \\ &=\frac{-200}{1,700} \times 100 \\ &=-11.76 \\ \% \text { change in price } &=\frac{130-120}{(130+120) / 2} \times 100 \\ &=\frac{10}{125} \times 100 \\ &=8.0 \end{aligned}