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
whereQdpr is the number of applicants to private universities per year in thousands, Ppris the average price of private universities (in thousands of USD), I is the household monthly income (in thousands of USD), and Ppu 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 Ppu is equal to 18.
The price elasticity of demand for private universities is closest to:
选项：
A. –3.1.
B. –1.9.
C. 0.6.
解释：
B is correct.
From the demand function:
ΔQdpr/ΔPpr=−3.1
Solve for Qdpr = 8r−3.1Ppr+0.8I+0.9Ppu
=84−3.1(38)+0.8(100)+0.9(18)=62.4
At Ppr = 38, price elasticity of demand=(ΔQdpr/ΔPpr)(Ppr/Qdpr)=(-3.1)(38/62.4)=-1.9
考点：需求价格弹性
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 = 8r−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价格弹性公示求解，即，
(ΔQdpr/ΔPpr)(Ppr/Qdpr)=(-3.1)(38/62.4)=-1.9
-3.1是怎么算出来的