Three ducts M, N and P can fill a tank in 18 hours. After working at it together for 6 hours, P is closed and M and N can fill the remaining part in 21 hours. The number of hours taken by P alone to fill the reservoir is:
42 hrs
M+ N + P = 118 For 6 hours=6×118 = 13 Remaining =23 (M + N)'s 21 hours work =23 (M + N)'s 1 hours work =121×23=263 P's 1 hour work = (M + N + P)'s work - (M + N)'s work = 118−263=142 P alone can fill the tank in 42 hours.