One way to look at this is to imagine both pipes connected to 2 identical tanks.
Consider the pipes to have started draining tanks at the same time.
In 36 hours, pipe B will drain one tank.
In the first 24 hours, pipe A will have drained the other tank
and would be able to drain another half tank in the last 12 hours.
Hence, the taps operating simultaneously could drain 2.5 tanks in 36 hours.
How long will it take to drain one tank ?
Yet another way is...
in 12 hours, one pipe will have drained half the tank
while the other pipe will have drained a third.
That's 3 sixths and 2 sixths.
In 12 hours, 5 sixths will be drained.
How much longer for another sixth ?