问题如下：
We observe the demand function for food is given by the equation
Qdpd =80−3Ppf+0.1I+0.5Ppd
Qdpd is the number of food, Ppd is the price of food, I is the income , Ppf is the price of drink
We know that:Ppf= 30, I=110, and Ppd= 20.
The price elasticity of demand for food will be:
选项：
A.–10.
B.–8.2.
C.0.6.
解释：
B is correct.
From above function, we get:
ΔQdpd/ΔPpd=−3
Qdpd = 80−3∗30+0.1∗110+0.5∗20=11
price elasticity of demand= (-3)(30/11)= -8.2
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，先求出需求量，即：
Qdpf = 80−3∗30+0.1∗110+0.5∗20=11
之后我们再利用需求价格弹性公示求解，即
(−3)(30/11)=−8.2
题目里的d和f好像是反了，需要改正。
