A tank can be filled by three pipes A, B, C in 12 hrs, 20 hrs and 16 hrs respectively. Each pipe is open for one hour alternatively starting with A and followed by B and C respectively, if a drainage pipe is open alone for every seventh hour then tank will be filled in 22 hour and 40 minute. Find in how many hours the drainage pipe can empty the tank alone?

In 21 hours (3 cycles of 7 hours each): A, B, C work for 3 hours each, drainage works for 3 hours. Net work = 3*(1/12 + 1/20 + 1/16) - 3/D = 43/60 - 3/D. Remaining work in hour 22 (A) + 23 (B): 1/12 + 1/20 = 8/60. Total work = 43/60 + 8/60 = 51/60. Remaining 9/60 done by C in 24th hour, but drainage opens in 22nd hour. The correct drainage rate is 40/3 hrs (approximately 13.33), which is not in options A-D.