An element X has a half life of 12.5 yr. Initially, it has N nuclei in its sample. How much of the sample is decayed after 25 yrs?

N/4

After 12.5 yrs, half the sample will be decayed and half will be left.

     Sample decayed : N/2

     Sample remaining : N/2

After another 12.5, half the sample will be decayed and half will be left.

     Sample decayed :\(\frac{1}{2}\) (N/2) = N/4

     Sample remaining : \(\frac{1}{2}\) (N/2) = N/4