[tex]\tt \large{\boxed{\bold{Nt=No.(\frac{1}{2}) ^{\frac{T}{t1/2} } }}[/tex]
Nt/No = 1/8
T = duration of decay
t 1/2 = half-life
[tex]\tt \dfrac{1}{8}=\dfrac{1}{2}^\dfrac{T}{5}}\\\\(\dfrac{1}{2})^3=\dfrac{1}{2}^\dfrac{T}{5}}\\\\3=\dfrac{T}{5}\rightarrow T=15~years[/tex]
