Question

Radioactive decay of 40k atoms in an igneous rock has resulted in a radio of 25 percent 40k atoms to 75 percent 40Ar and 40Ca atoms. How many years old is this rock

Answer 1

In order to answer this, we would need to know
the half-life of ⁴⁰K.  Perhaps when you look that up,
you'll be able to answer this on your own.