A substance has half of 3 min. After 6 min, the count rate was observed…

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

Correct Answer: Option A

Explanation

Let the count rate at zero time be N 

N (frac{3 min}{to})  (frac{N}{2}) (frac{3 min}{to}) (frac{N}{4})

(frac{N}{4}) 

= 400

N = (frac{400}{N/4}) x N 

= 1600

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