Answer to Question 3

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We need to know how many half lives gives 5 %

 

(0.5)y = 0.05  

 

y log10 (0.5) = log10 (0.05)

 

y -0.3010 = - 1.3010

 

y = -1.3010 -0.3010 = 4.32 half lives

 

Time taken = 4.32 28 years

 

Time taken = 121 years

 

 

 

You can use natural logarithms:

(0.5)y = 0.05  

 

y loge (0.5) = loge (0.05)

 

y -0.6931 = - 2.996

 

y = -2.996 0.6931 = 4.32 half lives

 

Time taken = 4.32 28 years

 

Time taken = 121 years