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The half life of a radioactive substance is 20 minutes. The approximate time interval (t2 - t1) between the time t2 when 2/3 of it had decayed and time t1 when 1/3 of it had decayed is:
- 14 min
- 20 min
- 28 min
- 7 min
Correct answer: 20 min
Solution
The half-life of a substance is the time required for half of it to decay. Since it takes one half-life (20 minutes) for the substance to decay from its original amount to half, and another half-life (20 minutes) to decay from half to one quarter, the total time interval between when 1/3 has decayed (2/3 remaining) and when 2/3 has decayed (1/3 remaining) is indeed 20 minutes.
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