L.F. Richardson proposed a model to describe the spread of war fever. † His proposal: if y = f(t) is the percent of the population advocating war at time t, then the rate of change of f(t) at any time is proportional to the product of the percentage of the population advocating war and the percentage not advocating war. Set up a differential equation that is satisfied by y = f(t). (Note: Use k for the constant of proportionality.)