A large tank is partially filled with 100 gallons of fluid in which 10 pounds of salt is...

A large tank is partially filled with 100 gallons of fluid in which 10 pounds of salt is dissolved. Brine containing 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