7. A fish tank initially contains 10 liters of a salt solution and contains 10 grams of salt mixed in the solution. Thereafter, pure water is pumped into the tank at a rate of 4 liters per minute, while the well mixed solution in the tank is pumped out at a rate of 3 liters per minute. Let x denote the mass of salt in the solution at time t. Find an initial value problem that is satisfied by x. That is, give a differential equation and an initial value. DO NOT SOLVE THE INITIAL VALUE PROBLEM.