a) 12.45 A.M.
b) 5 P.M.
c) 11.45 A.M.
d) 12 P.M.

Answer: c
Explanation:

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.