[tex]m(final)=m(initial)*( \frac{1}{2} )^{ \frac{time}{half-life}
m(final)= (1/8) *m(initial)
[/tex]
[tex] \frac{1}{8} m(initial)=m(initial)*( \frac{1}{2})^{ \frac{time}{halg-life} }
[/tex]
[tex] \frac{1}{8} = \left( \frac{1}{2} \right)^{\frac{time}{30} }
[/tex]
[tex]( \frac{1}{2})^{3}= (\frac{1}{2} )^{ \frac{time}{30}}
\\ \\ 3= \frac{time}{30}
\\ \\ time = 90 (years)[/tex]
