A large tank is filled to capacity with 300 gallons of pure water. Brine containing 4 pounds of salt per gallon is pumped into the tank at a rate of 3 gal/min. The well-mixed solution is pumped out at a rate of 6 gals/min. Find the number A(t) of pounds of salt in the tank at time t. How long (in minutes) will it take for the tank to be empty after this process has started?

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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A large tank is filled to capacity with 300 gallons of pure water. Brine containing 4 pounds of salt per gallon is pumped into the tank at a rate of 3 gal/min. The well-mixed solution is pumped out at a rate of 6 gals/min. Find the number A(t) of pounds of salt in the tank at time t. How long (in minutes) will it take for the tank to be empty after this process has started?

Expert Solution
Formation of ODE

Tank has been filled to capacity with 300 gallons of pure water.

Incoming rate of brine solution  is = 3 gal/min. and  it has  quantity 4 pounds/gal.

Outgoing rate of mixed solution = 6 gal/min.

A(t)  denotes the number of salt (pounds ) in the tank at time t.

 

The model can be written as ,

dAdt= input rate of solution - output rate 

input rate =4 lbgal ×3 galmin  = 12  lb/minoutput rate =6  galmin× A  lb300+t( 3-6)=6A 300-3t  lb/min

 

Therefore the  differentiation equation becomes ,

dAdt= 12 -6A300-3t with initial conditions A(0)=0 .

Or, dAdt+ 2A100-t=12 with A(0)=0

 

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