Final answer:
The price elasticity of demand is 2.2.
Step-by-step explanation:
The price elasticity of demand measures the responsiveness of quantity demanded to changes in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price.
In this case, the price rises from $8 to $9 (a 12.5% increase) and the quantity rises from 50 to 70 (a 40% increase). So, the price elasticity of demand is:
Elasticity = (% change in quantity demanded) / (% change in price) = (40% - 12.5%) / (12.5%) = 2.2.