Question 10
The beavers have built a water-purification machine for their city's water supply. To complete the 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 valves 2 to 6, how many can the beavers open to be certain that the water coming out has been filtered in the correct sequence?
Valve 1 is open and all the other valves are closed. Of valves 2 to 6, how many can the beavers open to be certain that the water coming out has been filtered in the correct sequence?