Final answer:
The price elasticity of demand (based on the midpoint formula) when the price decreases from 9 to 7 is 0.25.
Step-by-step explanation:
The price elasticity of demand (based on the midpoint formula) when the price decreases from 9 to 7 can be calculated using the following formula:
Elasticity = (Q2 - Q1) / [(Q2 + Q1) / 2] / (P2 - P1) / [(P2 + P1) / 2]
In this case, Q1 = 9, Q2 = 7, P1 = 9, and P2 = 7.
Plugging in these values, we get:
Elasticity = (7 - 9) / [(7 + 9) / 2] / (7 - 9) / [(7 + 9) / 2]
Simplifying this expression, we find:
Elasticity = -2 / 8 / -2 / 8 = 0.25
Therefore, the price elasticity of demand when the price decreases from 9 to 7 is 0.25.