✔ 最佳答案
Consider: Ed = % Change in Qd / % Change in price
For a, if price falls, Qd rises
It only states out the law of demand: an increase ( decrease ) in price of a good will result in a decrease ( increase ) in its Qd, ceteris paribus. From that, you cannot deduce the price elasticity of demand. So you are correct.
For b and c, there are some problems.
Ed refers to % change in Qd as well as % change in price, but not the absolute changes. Therefore the answers should be not enough information as well.
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Consider the case in b, suppose the original Qd is 90 and the new Qd is 100, % change in Qd:
[( 100 - 90 ) / 90 ] x 100% = 11.1%
Then Ed = 11.1% / 20% = 5 / 9 < 1
In this case, demand will be inelastic.
But then, if the orignal Qd is 10 and the new one is 20, the % change will be:
[( 20 - 10 ) / 10 ] x 100% = 100%
Then Ed = 100% / 20% = 5 > 1
In this case, demand will be elastic.
From the above, you can see that without the figures of % change, you can never tell what Ed is. Similar approach can be applied to c) and you will also notice that Ed cannot be determined.
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For d, it is correct because total expenditure = Qd x price, when Ed = 1, % change in Qd = % change in price and hence the total expenditure remains the same under the price change.
For e, apply the equation,
Ed = 20% / 10% = 2 > 1
Therefore it is of elastic demand.
2007-10-03 13:38:47 補充：
In the above, the absolute changes refer to b) Qd rises by 10 units ; c) price falls by 20 cents and Qd rises by 10 units.
