A large tank contains 130 litres of water in which 25 grams of salt is dissolved. Brine containing 10 grams of salt per litre is pumped into the tank at a rate of 9 litres per minute. The well mixed solution is pumped out of the tank at a rate of 4 litres per minute. (a) Find an expression for the amount of water in the tank after t minutes. (b) Let x(t) be the amount of salt in the tank after t minutes. Which of the following is a differential equation for x(t)?