Using optimization from calculus, how can this be solved?

A commuter train carries 2000 passengers daily from a suburb to the city. The cost to ride is $7.00 per person. Market research shows that 40 fewer people would ride the train for each $0. 10 increase in the fare, and 40 more people would ride the train for each $0.10 decrease. If the capacity of the train is 2600 passengers, and contracts with the rail employees require that at least 1600 passengers be carried, what fare should the railway charge to get the largest possible revenue