🚀 go-pugleaf

I'm not sure this is the right place to ask, but I will try anyways. I
attend an upper secondary school, and my teacher has just given us this:

Someone has a bath tub, and it is to be filled with water. If you tap cold
water, it takes 9 minutes to fill it. If you tap warm water, it takes 12
minutes. How much time will it take if you use both warm and cold water?

In my opinion there are more than one solution to this, because it depends
on the bath tub. (Are there one or two taps, does the water run in the same
speed or twice as fast when both warm and cold water is on?)

I have found one solution yet (two taps, double speed):

I set the size of the bath tub to 300 liters.
Cold water: 33,33 liters/minute
Warm water: 25 liters/minute

33,33x+25x=300
x=5,14

So, it would take 5 minutes and 8 seconds to fill the bath tub. Is this
right, and what are the other ways to do it? This is probably easy for you
skilled people, but not for me... Thanks in advance!

--
~Mmm
*http://privat.riksnett.no/~arve.moltubakk/mmm/*