Question 203502
If the empty tank can be filled in 45 minutes, then 1/45 of the tank can be filled in 1 minute.
If the full tank can be emptied in 30 minutes, then 1/30 of the tank can be emptied in 1 minute.
With both pipes open, then (1/45 - 1/30) of the full tank can be emptied in 1 minute.
{{{1/45-1/30 = -1/90}}} of the tank can be emptied (negative) in 1 minute.
So this means it will take 90 minutes to empty the tank completely.