Question 192007
1 / (1/30 - 1/45) = 90 minutes

Let me explain,

If the full tank can be emptied in 30 minutes then 1/30 of the tank can be emptied in a minute.
If the empty tank can be filled in 45 minutes then 1/45 of the tank can be filled in a minute.
When the two pipes are open together then 1/30 - 1/45 = 1/90 of the tank is emtied in a minute.
Therefore it takes 90 minutes for the full tank to be emptied when the two pipes are open at the same time.