A tank can be filled with water by two pipes A and B together in 36 hours. If the pipe B was stopped after 30 hours, the tank is filled in 40 hours. The pipe B can alone fill the tank in

Combined rate of A and B: 1/36 tank per hour. In first scenario: A+B work for 30 hours, then A works alone for 10 hours (total 40 hours). So 30(1/36) + 10(1/A) = 1 tank, where 1/A is A's rate. This gives 5/6 + 10/A = 1, so 10/A = 1/6 and A alone takes 60 hours. Since 1/A + 1/B = 1/36, we have 1/60 + 1/B = 1/36, giving 1/B = 1/36 - 1/60 = 1/90. So B alone takes 90 hours. Option D is correct.