Grade 11 physics question?

Water flows at 18 km/h at the top of a 40 m tall waterfall. If the rate of water flow is 5000 L/min, a) what is the total energy available from this waterfall per second? b) a generator is built at the base of the falls, which all of the water flows through, if this generator produces 4000 W of power, then what is the generator's efficiency?