A sample contains two isotopes -one stale and the other unstable. Number of stable nuclei is equal to `N_(s)` and number of unstable nuclei is equal to `N_(u)`. After a time the activity of the sample decreased to one thrid of the, the initial activity while the total number of nuclei (excluding decayed nuclei) became half. find the initial value of the ratio `N_(s)//N_(u)`.