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Question:

For a radioactive material, half-life is 10 minutes. If initially there are 600 nuclei, the time taken (in minutes) for the disintegration of 450 nuclei is?

30

20

15

10

Solution:

Half life = 10 min = ln2/λ
Nuclei disintegrate = 450
Nuclei left = 600 - 450 = 150
N = N₀e⁻λt
150 = 600e⁻(ln2/10)t
So, t = 20 minutes