18. The half-life of the radioactive isotope 90Sr, which enters the atmosphere during nuclear tests, is 28.1 years. Let us assume that the body of a newborn child has absorbed 1.00 mg of this isotope. How much strontium will remain in the body after
a) 18 years old,
b) 70 years, if we assume that it is not excreted from the body?

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