Instead of doing your problem, I'll do one exactly like it with different
numbers.
A tap can fill the tank in 15 minutes and another can empty in 6 minutes. If the
tank is already half full and both the taps are opened together,the tank will be:
The filling tap's rate is 1/15 of a tank per minute.
The emptying tap's rate is 1/6 of a tank per minute, which we interpret as a
negative filling rate, that is, a filling rate of -1/6 tankj per minute.
Therefore the combined rate of both taps is 1/15-1/6 = 2/30-5/30 = -3/30 or
-1/10 tank per minute. So we know the tank will be empty, since the combined
filling rate is negative. So we use the formula
TIME = (part of tank to fill, which is negative)/rate
TIME = (-1/2)/(-1/10) = (-1/2)(-10/1) = 5 minutes to empty it.
Now, you can use this as a model to solve your problem.
Edwin