Pipe A can fill the tank in 8 hours and pipe B can fill it in 12 hours. If pipe A is opened at 7:00 AM and pipe B is opened at 9:00 AM, then at what time will the tank be full ?

Answer: Option D

A opened 2 hours early to B
In 2 hours A can do 3 × 2 = 6 unit work
Remaining work = 24 - 6 = 18
A + B can do it in
$$\begin{array}{rl}& =\frac{18}{5}\text{hours}\\ & =3\frac{3}{5}\text{hours}\\ & \text{ = 3}\text{hours 36 minutes}\end{array}$$
∴ Tank will be full in 9 AM + 3 hours 36 minutes = 12.36 PM