Network Flow Problem; Node with no variable/constant coming in

Question

I have a linear algebra problem and I don't understand how I am suppose to solve the problem if there is no constant or variable coming in the St-Jean node and no constant or variable going out of the Moncton node. I learned that everything that comes in must comes out (and vice-versa), but what if nothing comes in or goes out? The problem also states that an average of 20 freight cars per day travel from Sherbrooke to Saint-John and 40 freight cars per day travel from Montreal to Quebec City. Thank you!