问题如下：
We observe the demand function for food is given by the equation
Qdpf =80−3Ppf+0.1I+0.5Ppd
Qdpf is the number of food, Ppf is the price of food, I is the income , Ppd is the price of drink)
We know that:Ppf= 30, I=110, and Ppd= 20.
The cross-price elasticity of demand for food will be:
选项：
A. 0.9.
B. 0.6.
C. 3.
解释：
A is correct.
From above function:
ΔQdpf/ΔPpd=0.5
Qdpf = 80−3∗30+0.1∗110+0.5∗20=11
Ppd= 20, the cross-price elasticity of demand will be 0.5(20/11)=0.9
考点：交叉价格弹性
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
之后我们再利用交叉价格弹性公式求解，即，
0.5(20/11)=0.9
对cross会有几种提问方式呢，怎么区分用谁的q和谁的p
