Model the situation using a differential equation. State the units of each variable and the units of the derivative.
A beach is eroding by 10 percent per year. Every month the waves deposit an extra 150 cubic feet of sand.
Let S be the amount of sand on the beach, measured in cubic feet. Let t be time in years. The rate at which sand accumulates is 150 cubic feet per year. The rate at which sand is carried away is 10 percent of the current amount of sand per year, or .1S. Putting these together,