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A substance has a half-life of 3min…

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A substance has a half-life of 3min. After 6min, the count rate was observed to be 400. What was its count rate at zero time?

A. 1600

B. 1200

C. 200

D. 2400




3min = half-life,

6min = half of half-life = 400

If the half of half-life has a count rate of 400, it then follows that 3min earlier, the count rate was twice.

So, 3min = 400 × 2 = 800

0 min = 800 × 2 = 1600

Now for the right answer to the above question:

  1. Option A is correct. 1600
  2. Option B is incorrect.
  3. C is incorrect.
  4. D is not the correct answer.


You may please note these/this:

  • Simple understanding of what half-life is all about will help you trace the initial count rate.
  • Half-life simply means the time taken for half of a radioactive substance to decay.
  • Half-life for the substance above is 3min, after 6min, 400 counts remained, this implies that at 3min, 800 would have remained, and at the beginning, 1600 would have remained.

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/ culled from 2020 JAMB-UTME physics past question 40 /

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  1. 0 Votes Thumb up 0 Votes Thumb down 0 Votes

    This implies that as the time reduces, the count rate increases. Is there really such a time as 0 mins for any substance? i guess the substance must have been existing before 0 mins.

    Avatar Yolala Ayuba - Sep 13, 2021 | Reply

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