Question 230679
It doesn't really matter how big the tank is,
because the 2 pipes will just get bigger and
bigger to fill or empty a bigger tank.
I'll say the tank holds 90 gallons when it's full
The rate of filling an empty tank is:
{{{90/45}}} gallons/min
The rate of emptying a full tank is:
{{{-(90/30)}}} gallons/min
(the minus sign is the only way I can 
tell if I'm emptying of filling)
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If both outlet and inlet are opened, I simply
add the 2 rates, still using the minus for emptying
{{{90/45 - 90/30 = r}}} gallons/min
{{{2 - 3 = r}}}
{{{r = -1}}} gallons/min
The tank is emptying at this rate with both pipes open
{{{90/-1 = -90}}} min
With both pipes open, the 90 gallon tank 
empties in 90 min