Initially 5 grams of salt are dissolved into 30 liters of water. Brine with concentration of salt 5 grams per liter is added at a rate of 3 liters per minute. The tank is well mixed and drained at 3 liters per minute.

1. Let $x$ be the amount of salt, in grams, in the solution after $t$ minutes have elapsed. Find a formula for the rate of change in the amount of salt, $dx/dt$, in terms of the amount of salt in the solution $x$.
   $\dfrac{dx}{dt} =$ grams/minute
   
   
2. Find a formula for the amount of salt, in grams, after $t$ minutes have elapsed.
   $x(t) =$ grams
   
   
3. How long must the process continue until there are exactly 25 grams of salt in the tank?
   minutes