A sample of a radioactive isotope used in medical diagnostics has a half-life of TTT. During a storage period of t=6Tt = 6Tt=6T, a total of Nd N_d\,Nd nuclei decay.
Which of the following represents the number of active (undecayed) nuclei N N\,N remaining in the sample at the end of this storage period?
Nd64\frac{N_d}{64}64Nd
Nd63\frac{N_d}{63}63Nd
Nd31\frac{N_d}{31}31Nd
63Nd63 N_d63Nd
