During the nuclear explosion, one of the products is ^90 Sr with half life of 6.93 years. If 1 μ g of ^90 Sr was absorbed in the bones of a newly…

During the nuclear explosion, one of the products is $\displaystyle { }^{90} \mathrm{Sr}$ with half life of $\displaystyle 6.93$ years. If $\displaystyle 1 \mu \mathrm{~g}$ of $\displaystyle { }^{90} \mathrm{Sr}$ was absorbed in the bones of a newly born baby in place of Ca , how much time, in years, is required to reduce it by $\displaystyle 90$% if it is not lost metabolically \_\_\_\_.