note:- there is some problem with this system therefor you will get answer with the expression I put here, to overcome this just click the edit button and correct the texture. Price elasticity of demand:- It simply give us the measure of the percentage change in the quantity demanded of any commodity corresponding to the given percentage change in the price of that commodity, keeping all other things constant . In lay mans' language it simply give us the responsiveness of the demand of any good X to any change in the Price of that good i.e., Px. % change in quantity demanded of X Price elasticity of demand = ------ % change in the price of commodity X Cross Elasticity Of Demand:- it measure the % change in the quantity demeaned of X good w.r.t. Change in the % change in the price of related good say, Y keeping all other things constant . % change in quantity demanded of X Cross Elasticity Of Demand = ------ % change in the price of commodity Y Income Elasticity Of Demand:- like the earlier two it is another measure of the flexibility of demand for any good, but now w.r.t. Change in the income of the consumer, keeping all other things constant. % change in quantity demanded of X Income Elasticity Of Demand = ------ % change in the income of the consumer Also In What Respect Would You Expect Determinant Of Demand For Computers To Differ From The Determinants Of Demand For Milk? answer to this 'Q' need simply the classification of the goods or specification the nature of the commodity. This I will answer latter, not 'coz I didn't know; but due to the the scarcity of time. Meanwhile you may consult the following books:- 1. Lipsey and chrystal, "Economics " any edition. 2. Samuelson , "Economics" any edition. 3. Koutsoyannis, "Microeconomics."