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 $$\to \frac{18}{5}$$
= $$3\frac{3}{5}\text{ hrs}$$
= 3 hrs 36 min
∴ Tank will be full in 9 am + 3 hrs 36 min = 12 : 36 pm