6. A tank contains 100-gallon of pure water. At time t = 0, a solution containing 2 lb of salt per gallon flows into the tank at a rate of 3 gallons per minute, and the well-stirred mixture flows out at a rate of 4 gallons per minute. Find the amount (in lb) of salt Q in the solution as a function of t in minutes.

Graph the line that represents a proportional relationship between d and t with the property that an increase of 33 units in t corresponds to an increase of 44 units in d. what is the unit rate of change of dd with respect to t? (that is, a change of 11 unit in t will correspond to a change of how many units in d? )