Total number of DNA molecules after environmental change

[Medicol]

1. Suppose DNA molecules of a Streptococcus bacterium contain a radioactive isotope $N^{15}$, if this bacterium is then put in an environment containing only $N^{14}$ and after 5 generations (DNA duplication), how many $N^{15}$ DNA molecules remain ?

Homework Equations

3. after 5 generations, it is $2^{5+1}=64$. But then I become clueless as to how I can reason the eliminated DNAs as well as their remaining number. Oh nooo...Help please...

 

[mfb]

I don't see why the total content of N¹⁵ should change at all. It just gets splitted and distributed over multiple bacteria.