B. 28 hours
Feedback The part of the tank filled in 1 hour = 1/12 + 1/24
Then, the part filled in 3 hours = 3[1/12 + 1/24] = 3/8
Remaining part to be filled = 1 - 3/8 = 5/8
Net part filled in 1 hour when A, B & C are opened = 1/12 + 1/24 - 1/10 = 1/40
Now, 1/40 part is filled in 1 hour then 5/8 part will be filled in [40 x 5/8] = 25 hours
Therefore, 3/8 part will be filled in 3 hours and 5/8 part filled in 25 hours.
Hence the total time taken to full the tank is 28 hours.