Microeconomics

Your computation of the market clearing price is correct. The equilibrium quantity can be derived by substituting this price into either the demand or the supply curve. Your choice. Hence

Q=300-2*90+4*25 <=> Q=220;

or equivalently

Q=3*90-50 <=> Q=220,

which is of course also a check that you computed the correct equilibrium price.

The price elasticity of demand is given by the formular

ε_p=p/Q*ΔQ/Δp

Find the demanded quantities with p=80 and p=100

p=80: Q=300-2*80+4*25 <=> Q=240

p=100: Q=300-2*100+4*25 <=> Q=200

This gives us

p=80, Q=240 and ΔQ/Δp=-40/20.

Using these numbers we find that

ε_p=80/240*(-40/20) <=> ε_p=-2/3

which says that as price increases by 1 % the quantity demanded will decrease by 2/3 %.

Hey, thanks a lot. Ir really helped!
 

The last sentence in #0: And how do I calculate the price elasticity of demand if I know that the price is $80 and $100. Don't I need more information? It's actually another assignment :)
 

Have a nice day

... and why do you use 240 instead of 200 when u are finding the quantity demanded with p=80 and p=100?

(just trying to get it)

#2 You cannot compute price elasticities with prices alone so if that problem is from another assignment, more information is needed. 

#3 I just assumed that p=80 was the initial price which increases to p=100. As you see from the formular, you must use information about the initial price, the corresponding quantity demanded as well as the changes in both price and quantity.

thanks