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# A count rate meter shows a count of 240 per minute from a given radioactive source. One hour later the meter shows a count rate of 30 per minute. The half-life of the source is?80min120min20min30min

A
80min
B
30min
C
120min
D
20min
Solution
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#### The number of counts left after time tN=N0(12)t/T1/2∴30=240(12)60/T1/2[∵t=1h=60min]or 30240=(12)60/T1/2or (12)3=(12)60/T1/2Comparing the powers, we get∴60T1/2=3T1/2=603or T1/2=20min.

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