🚀 go-pugleaf

In article <6tgc6c$rqg$1@readme.online.no>, Marie M. Moltubakk
<mamoltub@online.no> writes
>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/*
>
>
The method is fine but why choose a particular value for the volume?
If your answer depends on this value then it is obviously wrong.

So if volume of bath = V,
Cold water rate = V/9
Hot  water rate = V/12

Therefore rate of both together =
V/9 + V/12 = (4 X V + 3 X V)/36 = 7 X V/36

Hence time to fill the bath = V / (7 X V/36)
= 36 / 7 minutes = 5 minutes & 60/7 seconds
i.e. about 5 minutes and 9 seconds

-- 
Jeremy Boden