Fermentation Reactors - Medical Science

Q1:

In a continuous reactor, the medium contains 40 g.l-1 of maltose and the medium flow rate is 10 litres per hour and the effluent contains 20 g.l-l of lactate. What is the productivity of lactate production from this reactor?

A 50 g maltose.l-1

B 50 g lactate.l-1

C 200 g maltose.l-1

D 200 g lactate.l-1

ANS:A - 50 g maltose.l-1

Can anyone explain the method to solve this problem?