问题如下：
In a tourism-peak season,the demand of mineral water can be described as:
Qwd=30-2Pw
Qwd is the quantity of mineral water; is the price ). Pw is the price of mineral water If the mineral water price equals to 5, the price elasticity of demand will be:
选项：
A.–0.5.
B.–0.7.
C.–0.92.
解释：
A is correct.
When P =5,Q =30-2*5=20;
Price elasticity will be –2 × (5/20) = –0.5
考点：点弹性的计算。
解析：P =5时，带入公式可得Q =30-2*5=20;
依据点弹性公式可得：– 2 × (5/20) = – 0.5
请问这道题如何计算呢？
