P = .0025I - 0.5QD

The current market price is $11 and average income (I) is $10,000.

P =. 0025 I - 0.5Q P =11 Income =10,000

a)

Calculate the markets total Demand Market demand 11 = 0.0025 x10,000 - 0.5Q 0.5Q = 25 - 11 Q = 14/0.5 = 28

b)

Calculate the market's consumer surplus Consumer surplus = 1/2 x 28 x 4 = 112/2 = 56

c)

Calculate the price elasticity of demand Price elasticity = dQ/dP x P/Q P=25-0.5Q dP/dQ = 0.5 dQ/ dP =1/0.5 =2

Is the Price elasticity of demand Price elasticity = dQ/dP x P/Q = 2 x11/28 = 2x0.39 = 0.78

For part C, calculating price elasticity of demand, how do we arrive at "2"? I understand the rest, but not that part.