Mixtures type word problem

**50 g of a certain chemical is added to 200mL of water; the chemical dissolves in water at a rate proportional to the product of the amount of undissolved chemical and the difference between concentrations in a saturated solution and the existing concentration in the solution. A saturated solution contains 25g of chemical in 100mL of solution.**

**(a) Show that the differential equation that describes the situation is,**

$\displaystyle \frac{dx}{dt} = \frac{k}{200}(50-x)^{2}, x(0) = 0$

**where x(t) is the number of grams of dissolved chemical at time t.**

These word problems always cause me trouble. The question seems so complicated I don't even know what to start with.

Can someone help me get started?