A source contains two phosphorus radionuclides, and . Initially 10.0% of the decays come from . We have to find the time that one has to wait until 90.0% of the decays do so.

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A source contains two phosphorus radionuclides, and . We calculate first the decay constant for and radionuclides.

Let the initial number of and nuclides be and , respectively.

It is given that initially 10.0% of decays come from . We can express it in the form of an equation as follows:

Let the time for which one has to wait until 90.0% of the decays come from be .

We can express it as an equation using the radioactive decay law as follows:

That is for decays from nuclide to be 90.0% of the decays of the two nuclides one has to wait for 208 days, if initially it was 10.0%.