perhaps you are overthinking this, its a base math question 180 seconds / 6 = 30 u2026u2026u2026u2026u2026. 30 x 5 = 150. each pipes value is 30 seconds. if you remove 1 pipe from the system then you would have to add that 30 seconds to the other pipes .so. 210 ( 30 seconds ADDED to the filling time ) Divided by 5 = 42 .. each pipe s new value is 42 seconds and the total filling time will be 210 seconds or 3.5 minutes assuming all the pipes fill the cistern at an equal rate.