a pipe working at full speed can fill an empty tank in 2 hours. however, during the first hour it served one-twelfth of its capacity, during the second hour it served one-ninth of its capacity, during the third hour it served one-sixth of its capacity, during the fourth hour it worked at one-fourth of its capacity and during the fifth hour it worked at only one-third of the capacity at which it was supposed to operate. was efficient. the second pipe also did the same thing, but if it worked at its full speed, it had to fill the empty tank in 4 hours. if both these pipes and the pipe which empties the tank at a constant speed are turned on simultaneously, then half empty tank can be filled in 5 hours. if a single emptying pipe is run, how many hours will it take to empty the filled tank? (a) 30 (b) 32 (c) 20 (d) 24