the time that has been given to you in the question don't really mean a thing, other than to test your understanding of half-life. radioactive substance decay with a constant half-life. eg. if substance A decays radioactively to form substance B, and its half-life is 8days, then if you have 8g of substance A at day 1, then it means by 8th day, your sample would now contain 4g (ie. 1/2 of 8g) of substance A, but the sample you have now is a mixture of A and B. radioactive is a continuous decay phenomena.

are you familiar with calculus? radioactive decays is a first order rate equation, which you can integrate to obtain the decay profile.

radioactivity is directly proportional number of radioactive atoms

let N be number of radioactive atoms

dN/dt = -kN

(1/N)dN/dt = -k

integrate LHS and RHS with respect to time

ln N = -kt + C (constant of integration)

when t = 0, ln N_{o} = C where N_{o} is the initial number of radioactive atoms.

therefore, ln N - ln N_{o} = -kt => ln (N/N_{o}) = -kt

half-life of a substance (t_{1/2}) is given by:

N/N_{o} = 1/2 => - ln2 = -kt_{1/2}

t_{1/2} = (ln 2)/k

k = ln 2 / t_{1/2}

the decay profile is thus:

ln (N/N_{o}) = -kt

ln (N/N_{o}) = -(ln 2 / t_{1/2})t

thus by knowing the half life, you can calculate the decay constant (k) and thus obtain the fill decay profile of the substance of interest.