A radioactive isotope has a half life of T years. How long (in years) will it take the activity to reduce to 3.125% of it's original value ?

4T 5T 3T 6T

5T

$ N = \frac{N_0}{2^{\frac{t}{T}}}= \frac{N_0}{32} = \frac{N_0}{2^5}$ $ \frac{t}{T} = 5 \Rightarrow t = 5T$