Help on elasticity of demand please!?

Question

Qd = 90-2P and Qs= 4P.

After you set them equal, you get P=15 and Qs&Qd = 60.

The question is, The price of elasticity of demand at the equilibrium point is ____.

a, -2

b, -.5

c, -.5%

d, -4

e, -2%

I know the whole formula and concept; however, I am confused because I thought you needed TWO numbers to be able to find the change. Am I wrong and is there just a way to find price elasticity of demand at equilibrium with only one number?

My only guess is that this question refers to the question before, which asked a question with the price = 20.

However, I can't get a right answer for the price elasticity of supply at equilibrium point if this is the case (answers: a- 2, b-4, c-2%, d-10, e-1)

Any help would be greatly appreciated!

Answer 1

For demand:

b, -.5

E= (P/Q) * δQ/δP

P=15

Q=60

δQ/δP=(90-2P)' = -2

E= -2 * 15 / 60 = -30/60 = -0.5

For supply:

e-1

E= (P/Q) * δQ/δP

P=15

Q=60

δQ/δP=(4P)' = 4

E= 4*15/60 = 60/60 = 1