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Answer:
A = 232 mg
Step-by-step explanation:
First, we need to find the half-life (hl) of the isotope:
A = A02^-(t/hl) taking the natural logarithm of both sides
ln(A/A0) = -(t/hl)ln2 or
hl = -t ln2 /ln(A/A0)
= -(2 yrs)ln2 / ln(390/480)
= 6.68 yes
After 7 years, in 2016 we expect the mass of the isotope to be
A = (480mg)2^-(7/6.68)
= 232 mg