. In certain nuclear explosions the radioactive isotope 90Sr can be generated. The danger of this nuclide is that it may

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. In certain nuclear explosions the radioactive isotope 90Sr can
be generated. The danger of this nuclide is that it may be
incorporated into bones in place of calcium. 90Sr emits β rays of
energy 0.55 MeV. Its half-life (First order) is 28.1 years. Suppose
a child absorbs 1.00 µg of 90Sr. Assuming that none of the 90Sr is
used metabolically how much 90Sr will remain after
(a) 18 years
(b) 70 years?