A cistern can be filled by two pipes A and B in 12 and 15 hours respectively. If both are opened together, how long will it take to fill the cistern?

प्रति घंटे कार्य $= 1/12 + 1/15 = (5+4)/60 = 9/60 = 3/20$। लिया गया समय $= 20/3 = 6.67$ घंटे (6 घंटे 40 मिनट)।