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Onett

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## Homework Statement

A tank contains 80 gallons of pure water. A brine solution with 2 lb/gal of salt enters at 2 gal/min, and the well-stirred mixture leaves at the same rate. Find (a) the amount of salt in the tank at any time and (b) the time at which the brine leaving will contain 1 lb/gal of salt.

## Homework Equations

I'm just wondering about (b) really. I know we set S=80 below to solve it, but why?

## The Attempt at a Solution

The differential equation that gives (a) is

S=160 - 160*e^(-t/40)

where S is the amount salt in the tank at any time t.

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