Two substances R and S decompose in solution independently, both...

Two substances $\mathrm{R}$ and $\mathrm{S}$ decompose in solution independently, both following first order kinetics. The rate constant of $\mathrm{R}$ is twice that of $\mathrm{S}$. In an experiment, the solution initially contained 0.5 millimoles of $\mathrm{R}$ and 0.25 of $\mathrm{S}$. The molarities of $\mathrm{R}$ and $\mathrm{S}$ will be equal just at the end of time equal to