Law of Radioactive Decay question from NEET 2002 (AIPMT)

Answer: (c)

1. $n = \frac{30}{10} = 3$ half-lives (here 1016 means $10^{16}$).
2. Remaining $= \frac{4 \times 10^{16}}{2^3} = 0.5 \times 10^{16}$.
3. Decayed $= 4 \times 10^{16} - 0.5 \times 10^{16} = 3.5 \times 10^{16}$.
4. So the answer is (c) $3.5 \times 10^{16}$.