NO.PZ2016010802000025
问题如下：
The market demand function for four-year private universities is given by the equation
Qdpr =84−3.1Ppr+0.8I+0.9Ppu
where Q pr d is the number of applicants to private universities per year in thousands, P pr is the average price of private universities (in thousands of USD), I is the household monthly income (in thousands of USD), and P pu is the average price of public (government-supported) universities (in thousands of USD). Assume that Ppr is equal to 38, I is equal to 100, and P pu is equal to 18.
The income elasticity of demand for private universities is closest to:
选项：
A. 0.5.
B. 0.8.
C. 1.3.
解释：
C is correct.
From the demand function:
Solve for Qdpr :
∆Q /∆Idpr = 0.8 (coefcient in front of the income variable)
Qdpr = 84 – 3.1Ppr + 0.8I + 0.9Ppu
= 84 – 3.1(38) + 0.8(100) + 0.9(18)
= 62.4
At I = 100, the income elasticity of demand
(ΔQdpr/ΔI)(I/Qdpr)=(0.8)(100/62.4)=1.3
考点：需求收入弹性
解析：题目给出了需求函数的表达式，带入数字，先求的私立大学的需求量，即：
Qdpr = 84 – 3.1Ppr + 0.8I + 0.9Ppu
= 84 – 3.1(38) + 0.8(100) + 0.9(18)
= 62.4
之后我们再利用需求收入弹性公式求解，即，
(ΔQdpr/ΔI)(I/Qdpr)=(0.8)(100/62.4)=1.3
课件上写的是数量比收入～为什么解析是收入比数量？