After four half-lives of decay, what is the ratio of radioactive parent isotope to stable daughter isotope?

Ratio of parent to daughter nuclei is 16 : 1

OR

1 : 0.0625

Explanation:

The half of a radioisotope is the time it will take half of the original mass to decay.

Let the original mass be y

After the first half life, the new mass will be y/2

After the second half decay, the new mass will be y/4

After the third half decay, the new mass will be y/8

After the fourth half life decay, the new mass will be y/16.

Thus the ratio of the mother nucleus to the daughter nucleus will be y : y/16

Multiply the ratio through by 16

= 16y : y

= 16 : 1