A bathtub with a capacity of 530L initially contains 125L of water with 500g of epsom salt dissolved in it. A solution with a salt concentration of 3g/L is added to the tank at a rate of 5L/min. The well-mixed solution drains out of the tank at a rate of 2L/min.
1. Write a differential equation that models the scenario.
2. Find a formula for the mass of salt in the bathtub at any time t
3. What is the concentration of salt in the bathtub when it starts to overflow?