它等于需求量变化除以
价格变动 i.e. (dQ ) / (dP)

dQ = (20000-30000) / ((20000 + 30000) / 2) = -0.4
dP = (11-9) / ((11+9)/2) = 0.2

dQ / dP = -0.4 / 0.2 = 2 = A

按照这里
https://opentextbc.ca/principlesofeconomics/chapter/5-1-price-elasticity-of-demand-and-price-elasticity-of-supply/#:~:text=The%20price%20elasticity%20of%20demand%20is%20calculated%20as%20the%20percentage,the%20percentage%20change%20in%20price.&text=Therefore%2C%20the%20elasticity%20of%20demand,is%20inelastic%20in%20this%20interval.