th The half-life of the isotope ₁₁Na²⁴ is 15 hr. How much time does it take for (7/8) of a sample of this isotope to decay? (a) 45 hour (b) 60 hour (c) 75 hour (d) 90 hour

Answer: (a). If 7/8 decays, 1/8 = (1/2)³ remains, i.e. 3 half-lives. t = 3×15 = 45 h. So the answer is (a) 45 hour.

th The half-life of the isotope ₁₁Na²⁴ is 15 hr. How much time does… | MHT CET 2026 (17 Apr, Shift 1)

Q7MHT CET 2026 · 17 APR · SHIFT 1Medium

Law of Radioactive Decay

th The half-life of the isotope ${}_{11}\text{Na}^{24}$ is 15 hr. How much time does it take for $\left(\frac{7}{8}\right)$ of a sample of this isotope to decay?

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Answer: (a)

If $\frac{7}{8}$ decays, $\frac{1}{8} = \left(\frac{1}{2}\right)^3$ remains, i.e. 3 half-lives.
$t = 3\times15 = 45$ h.
So the answer is (a) 45 hour.

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