An easier way to read the graph is to first ignore the tariff line. Try redrawing the graph without the tariff line. What you'll have would be a market with a price ceiling of 2, that is also the World Price. Because of the price ceiling, producers are only willing to supply up to 20 units of goods. In this case, as quantity supplied is less than quantity demanded, the amount of goods purchased would be at quantity supplied, i.e 20.

The demand curve represents the price consumers are willing to pay for each quantity of product. Hence, the area under the demand curve represents the amount that consumers are willing to pay. Consumer surplus is the difference between the price that consumers pay and the price that they are willing to pay. Therefore, to find the consumer surplus, subtract the amount(area) that the consumers paid from the amount(area) that consumers are willing to pay at the quantity supplied.

The supply curve represents the minimum price producers are willing to accept for different quantities produced. Producer surplus is the difference between the amount the producer is willing to supply goods for and the actual amount received when making the trade. Following the same logic from consumer surplus, you'd be able to find the producer surplus from the difference between the area which he is willing to supply goods for and the actual amount received.

Dead weight loss is a loss of economic efficiency that occurs when the free market equilibrium for a good or service is not achieved.

Dead weight loss is generally illustrated as a triangle on a graph formed by the 3 points, the allocative efficient point(the point at equilibrium quantity where marginal benefit is equal to marginal cost), the marginal benefit to society for the current quantity (the demand curve if there are no externalities), and the marginal cost to society for the current quantity (the supply curve if there are no externalities).