College Algebra & Trigonometry, 2018a

160 CHAPTER 3. EXPONENTS AND LOGARITHMS
15) Continuous Mixing
A tank contains 200 gallons of a 2% solution of HCl. A 5% solution of HCl is
added at 5 gallons per minute. The well mixed solution is being drained at 5
gallons per minute.
The amount of HCl in the tank at any given time t in minutes is:
y =10− 6e −0.025t
a) How many gallons of HCl are in the tank at t =25minutes?
b) When does the concentration of HCl in the solution reach 4%?
16) Continuous Mixing
A tank contains 100 gallons of salt water which contains a total of 25 lbs of salt.
Salt water containing 0.4 lbs per gallon is added to the tank at a rate of 5 gallons
per minute and the well-mixed solution is drained at the same rate.
The amount of salt in pounds in the tank at any given time t in minutes is:
y =40− 15e −0.05t
a) How many pounds of salt are in the tank at t =20minutes?
b) How long does it take for there to be 30 lbs of salt in the tank?