# Homework Help: Extracting the question (D.E.)

1. Feb 19, 2010

### annoymage

1. The problem statement, all variables and given/known data

A tank is filled with 10 gal (gallons) of brine in which 30 lb of salt is dissolved.
Brine having 2 lb of salt per gal enters the tank at the rate of 2 gal per min and the
well-stirred mixture leaves at the rate of 3 gal per min.

(a) Set up a differential equation fro the amount of salt at time t

2. Relevant equations

let S = be the salt leaves and enter
V = volume of the tank

3. The attempt at a solution

im actually stuck with the question, at the

"and the well-stirred mixture leaves at the rate of 3 gal per min."

im terrible with english, so, someone give me clue

2. Feb 19, 2010

### BishopUser

It just means that you need to assume that as soon as the new brine enters the tank, the concentration of salt in the tank is uniform (perfectly mixed). So the D.E. would simply just be = (rate that salt enters tank) - (rate that salt leaves tank).

3. Feb 19, 2010

### annoymage

owh yeaaaahhhhhhhhhhhhhhhhhhh, how come i miss the word "leaves"
ok thank you very much