Question 9
The beavers built a machine to clean the city’s water. To complete treatment, water must pass through filters N, P and C in that order.
Valve 1 is open and all the other valves are closed. Of the valves numbered 2–6, how many can the beavers open and be certain of receiving correctly treated water at that outlet?
Valve 1 is open and all the other valves are closed. Of the valves numbered 2–6, how many can the beavers open and be certain of receiving correctly treated water at that outlet?