is the rate of change of volume of water in the tank in liters per day.

Graph of  with respect to time is shown.

Amount of water in the tank at time , is L.

Amount of water in the tank after four days is .

Determine .

Use the Midpoint rule to estimate the total amount of water in the tank after four days. 

Mid point rule:

, where .

.

Sustitute corresponding values in above expression from the graph.

.

.

Substitute in  

.

Amount of water in the tank after four days is L

Amount of water in the tank after four days is L.