Suppose that radioactive element X decays into another element Y, which is stable. If the initial concentrations of element X and element Y are Xo and Yo, what are the equations that describe the...

Suppose that radioactive element X decays into another element Y, which is stable. If the initial concentrations of element X and element Y are Xo and Yo, what are the equations that describe the time-dependent concentrations of these two elements?