Let the initial number of radioactive nuclei be N0.
Then, after a half-life period, the number of nuclei present would be N0×12.
After two half-lives, the number of nuclei present would be 12×(N0×(12))=N0(12)2
Similarly, after ′n′ half-lives, the number of nuclei present would be N=N0(12)n →(1)
If ′t′ is the given time period and T is the half-life period of the radioactive substance,
then n=tT →(2)
Also, if ′m′0 is the initial mass and ′m′ the mass of undecayed nuclei of the substance,
then NN0=mm0 →(3)
∴ Equation (1) can be written as
m=m0(12)n or m=m0(12)tT →(4)
In the given problem, undecayed mass
m=m0−78m0=18m0 ; t=15 days, T= ?
Substituting the values of ′m′,′m′0 and ′t′ in equation (4), we get
18m0=m0(12)15T⇒18=(12)15T⇒(12)3=(12)15T
⇒15T=3⇒T=153 days =5 days