【How-to】How to calculate total surplus

Rewrite the demand and supply equation as P = 20 – Q and P = Q/3. With $4 tax on producers, the supply curve after tax is P = Q/3 + 4. Hence, the new equilibrium quantity after tax can be found from equating P = Q/3 + 4 and P = 20 – Q, so Q/3 + 4 = 20 – Q, which gives QT = 12.