Question

How many half-lives are required for the activity of radionuclide to decrease by a factor 64

Answer 1

By definition, during one half-life the activity of radionuclide decreases by a factor of 2 (see https://en.wikipedia.org/wiki/Half-life). SInce $64 = 2^6$, 6 half-lives are required for the activity of radionuclide to decrease by a factor 64.

Asnwer. 6.