First slide
Radio activity
Question

Half – life of a radioactive substance is 20min. The time between 20% and 80% decay will be

Easy
Solution

20 % decay, present 80%
80 % decay, present 20%
80 % to 20 % becomes ¼  
i.e., two half life time
i.e., 2 x 20 = 40 mins

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