Differential equations?

a tank initially contains 100gal of salt-water solution containing .05lb of salt for each gallon of water. at time zero, pure water is poured into the tank at a rate of 2 gal per min. simultaneously, a drain is opened at the bottom of the tank that allows the salt-water solution to leave the tank at 3 gal per minute. what will be the salt content in the tank when precisely 50 gal of salt-solution remain??