The demand function for a cola-type soft drink in general is Q = 20 - 2P, where Q stands for quantity and P stands for price.
a. Calculate point elasticities at prices of 5 and 9. Is the demand curve elastic or inelastic at these points?
b. Calculate arc elasticity at the interval between P = 5 and P = 6.
c. At which price would a change in price and quantity result in approximately no change in total revenue? Why?