A large tank is partially filled with 100 gallons of fluid in which 10 pounds of
salt is dissolved. Brine containing ½ pound of salt per gallon is pumped into
the tank at a rate of 6 gal/min. The well-mixed solution is then pumped out
at a slower rate of 4 gal/min. Find the number of pounds of salt in the tank
after 30 minutes.