The demand for rail travel is $Q^d = 600 - 2P$ where quantity is thousands of train journeys per quarter and $P$ is in £ per journey. How much would the consumer surplus change if rising cost of electricity led the train companies to raise price from £70 to £100.
Calculating the choke price, $600 = 2P$. Thefore $P = 300$.
At £70, $Q^d = 460$. Therefore Consumer surplus is $(460-300)/2*460 = £36,800$
At £100, $Q^d = 400$. Therefore consumer surplus is $(400-300)/2*400 = £20,000$
Difference is $£16,800$.
I've got the answer wrong, but i'm sure where, any help is appreciated.