For Question 13 below, the formula for the price elasticity of demand is: where the elasticityeis equal to the percent change in the quantity demanded (% ? Q_D) divided by the percent change in price (% ? P). As Cooter and Ulen explain (p.25, footnote 8),eis always stated as a positive number, by convention, even though the percent changes (e.g., quantity-demanded increasing as price decreases, or vice versa, according to the law of demand), divide to a negative. The full expression of that convention is simply the absolute value: