What is the Formula for Dilution with a Full Tank and Only Fresh Water Inflow?

[neshepard]

I have a problem I finally answered, but it made me think. The formula is dy/dt=rate in - rate out for dilution. If a tank is full of salt and only fresh water flows in, are we treating rate in as 0 if we are looking to find how much salt is left at any time t?

 

[Mark44]

It depends on what y represents. If y is the amount of salt or concentration of salt at time t, then yes, the rate in is zero.

 

[neshepard]

Yes, y represents the amount of salt and t is the time. So the only time there is a rate in for the formula is if there is actually, in this case salt, flowing in?

 

[Mark44]

Yes.