Discussion
What will be the radioactivity of 125I labelled 2 mCi thyroxine after 4 generations (t½ = 60 days) :
Answer: B
The remaining activity is 0.125 mCi.
'4 generations' here means four half-lives (4 × 60 = 240 days). Each half-life halves the activity:
\[ \begin{aligned} A &= A_0 \left(\tfrac{1}{2}\right)^{4} \\ &= 2 \times \tfrac{1}{16} = 0.125\ \text{mCi} \end{aligned} \]
0.25 mCi is the value after three half-lives and 1 mCi after only one. 0.125 µCi has the right number but the wrong unit, a thousand times too small.
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