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\begin{aligned} \% \text { change in quantity } &=\frac{\mathrm{Q}_{2}-\mathrm{Q}_{1}}{\left(\mathrm{Q}_{2}+\mathrm{Q}_{1}\right) / 2} \times 100 \\ \% \text { change in price } &=\frac{\mathrm{P}_{2} \mathrm{P}_{1}}{\left(\mathrm{P}_{2}+\mathrm{P}_{1}\right) / 2} \times 100 \end{aligned}