Pipe B fills the tank at a rate of 1/12 of a tank per hour.
At 1:30 pipe A has been running for 1.5 hours and so the tank is (1.5)/8
Now suppose it takes a further x hours to fill the tank, then we have an
additional x/8 of a tank added from A and x/12 of a tank from B. Hence as
the tank is now full we have:
(1.5)/8 + x/8 + x/12 = 1
Now solve this for x to find how many hours after 1:30 the tank is full, and
so the time at which it is full.