Question 9
To clean their city’s domestic water supply, the beavers have built a water filter. To complete treatment, water must pass through filters N, P and C in exactly that order.
Valve 1 is open and all the other valves are closed. Among valves 2 to 6, how many can the beavers open to be certain that the water from that outlet has been filtered in the correct sequence?
Valve 1 is open and all the other valves are closed. Among valves 2 to 6, how many can the beavers open to be certain that the water from that outlet has been filtered in the correct sequence?