Answer:
D. 62.9 days
Step-by-step explanation:
Half Life Cpm activity:
320,000 / 2 = 160,000
At 100 days cpm is 120,000 then cpm 160,000 will be at 62.9 days.
[320,000 - 216,100] / 40 days = 2,597.5
160,000 / 2,597.5 = 62.9 days.
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