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2003 Section 2 Question 15

15) 4 minutes

Initial count rate = 140 – 20 (background radiation) = 120
Final count rate = 35 – 20 = 15
We need to figure out the number of half lives.

120 → 60 = 1 half life

60 → 30 = 2 half lives

30 → 15 = 3 half lives

There were three half lives over the period of 12 minutes

Time per half life = 12 ÷ 3 = 4 minutes

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