A tank can be filled by pipe A in 2 hours and pipe B in 6 hours. At 10 A.M. pipe A was opened. At what time will the tank be filled if pipe B is opened at 11 A.M. ?

Answer: Option C

Pipe A will fill 3 units till 11 A.M. Remaining capacity
= 6 - 3
= 3 units
Now both pipes will fill the tank in
$$\frac{\text{Total Capacity}}{\text{Efficiency }}=\frac{3}{\left(3+1\right)}=\frac{3}{4}\text{ hours}$$
So, $$\left(11+\frac{3}{4}\right)$$  A.M., tank will be filled = 11.45 A.M.