A fish tank initially contains 30 liters of pure water. Brine of constant, but unknown, concentration of salt is flowing in at 3 liters per minute. The solution is mixed well and drained at 3 liters per minute.

1. Let $x$ be the amount of salt, in grams, in the fish tank after $t$ minutes have elapsed. Find a formula for the incremental change in the amount of salt, $\Delta x$, in terms of the amount of salt in the solution $x$, the unknown concentration of incoming brine $c$, and the incremental change in time $\Delta t$. Enter $\Delta t$ as Deltat.
   $\Delta x =$ grams
   
   
2. Find a formula for the amount of salt, in grams, after $t$ minutes have elapsed. Your answer should be in terms of $c$ and $t$.
   $x(t) =$ grams
   
   
3. In 15 minutes there are 25 grams of salt in the fish tank. What is the concentration of salt in the incoming brine?
   $c =$ g/L