Pipes A and B can fill a storage tank in 8 hours and 12 hours, respectively. With the tank empty, pipe A was turned on at noon, and then pipe B was turned on at 1:30pm. At what time was the tank full?

How do I solve this??

x= ???

I got x/8 = amount of the tank filled using A

x/12 = amount of tank filled using B

maybe 1.5x/8 + x/12 = 1 ?