Economics, 3rd Edition

Qd = 20 − 2p

Qs = 2 + 2p

In this case, the terms are all neatly aligned above each other so it is a relatively simple task to add the 

two together. Note that we are trying to find equilibrium so Qd

= Qs, thus the value of Q is the same. 

Adding the two together we get:

Qd = 20 − 2p

    

Qs = 2 + 2p

  

2Q = 22

  

  

Q = 11

Notice that in the above equations we have a very convenient fact that the coefficient of p in each case 

is the same but with opposite signs. This makes this example very easy to eliminate p to isolate the Q 

value. This is not always the case, however, but it is important to remember that having two equal values 

with opposite signs allows us to get rid of them! We will come back to this later.

We can now use the fact that we know Q to find the equilibrium price by substituting Q into one of the 

equations thus: