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

12:36 pm

From 7 am to 9 am work done by A only = 3 × 2 = 6

Remaining work = 24 - 6 = 18

Time at which tank will be filled by (A + B) = \(\frac{18}{3+2}\) = 3\(\frac{3}{5}\) + 9 am = 12:36 pm