one of radioactiviy's application - carbon 14 dating

     Carbon-14 dating 
     Carbon-14 dating is a way to for determining the age of ancient biological remains up 
     to about 50,000 years old. Carbon-14 is [radioactive], with a [half-life] of about 5,600 
     years. Plants absorb and give out carbon dioxide from their bodies regularly, and animals
     and people eat plants. The carbon content inside living things is quite constant, as well 
     as the ratio of normal carbon (carbon-12) to carbon-14. However, when an organism dies, 
     the carbon-14 inside keeps on decaying by [beta]emission while the amount of carbon-12 
     in that organism remains unchanged. By comparing the amount of carbon-12 to carbon-14, 
     or by measuring the activity of carbon-14, the age of an ancient organism can be determined.


    Question
     If the tissue of a living plant will give a count rate of 60 counts per minute, for how many 
     years has the plant been dead if the count rate had dropped to 15 counts per minute?



        


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