Because total consumer surplus has fallen by more than the tax revenue, the tax has a deadweight loss.
The deadweight loss of tax is the reduction in total surplus when a tax is levied on a good. Consumer surplus is the difference between the equilibrium price of a good and the value the consumer places on the good.
Initial consumer surplus = ($20 - $15) + ($17 - $15) = $7
New consumer surplus = ($20 - $18) = $2
Thus, there is a reduction in consumer surplus and there is a deadweight loss.
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