The half life of a radioactive nucleus is 

50 days. The time interval (t2-t1) between 

the time t2 when 23 of it has decayed and 

the time t1 when 13 of it had decayed is

(a) 30 days

(b) 50 days

(c) 60 days

(d) 15 days

Active fraction at instant t2,

                  12t2/T1/2=13

Active fraction at instant t1,

              12t1/T1/2=23

           2t2/T1/22t1/T1/2=22t2-t1T1/2=2t2-t1=T1/2=50 days

Difficulty Level:

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