We have not discussed methods by which systems of first-order differential equations can be solved. Nevertheless, systems such as dx dt = −λ1x dy dt = λ1x − λ2y dz dt = λ2y (which is discussed here) can be solved with no knowledge other than how to solve a single linear first-order equation. Suppose that time is measured in days, that the decay constants are k1 = −λ1 = −0.135229 and k2 = −λ2 = −0.004951, and that x(0) = 20, y(0) = 0, z(0) = 0. Use a graphing utility to obtain the graphs of the solutions x(t), y(t), and z(t) on the same set of coordinate axes. Use the graphs to approximate the half-lives of substances X and Y. (Round your answer to the nearest integer.) X days Y days