Question

The half life of a radioactive element is 20 hears. the percntage of the element will be left after 40 year is?

Answer 1

t1/2 = 20 years ( time to reduce by half of initial value i.e 50%)
now after 40 years it will decompose by 75%
                                                                            
as,  t75% = 2 .  t50%
       t75% = 2 .  20 =  40 years
so, percentage of the element left after 40 years will be 25%

hope , you got this