If 80% of a radioactive element undergoing decay is left over after a certain period of time / from the start, how many such periods should elapse from the start for justover 50% of the element to be left over?

Let α = 100, (a -x) = 80 ( Amount left)∴ Amount left =a(2)n [n= number of half lives ]80=100(2)n⇒(2)n=108∴nlog⁡2=log⁡10−3log⁡2=1−3×0.3=0.1n=0.1log⁡2=0.10.3=13.n=13=t Time for 80% amount left  or 20% decomposed t1/2 Time for 50% amount left  or decomposed ⇒t1/2=3t