Answer:
The approximate difference in the half-lives of the isotopes is 66 days. 
Step-by-step explanation:
The decay of an isotope is represented by the following differential equation:

Where:
 - Current mass of the isotope, measured in kilograms.
 - Current mass of the isotope, measured in kilograms.
 - Time, measured in days.
 - Time, measured in days.
 - Time constant, measured in days.
 - Time constant, measured in days.
The solution of the differential equation is:

Where  is the initial mass of the isotope, measure in kilograms.
 is the initial mass of the isotope, measure in kilograms.
Now, the time constant is cleared:


The half-life of a isotope ( ) as a function of time constant is:
) as a function of time constant is:


The half-life difference between isotope B and isotope A is:

If  ,
,  and
 and  , the difference in the half-lives of the isotopes is:
, the difference in the half-lives of the isotopes is:


The approximate difference in the half-lives of the isotopes is 66 days.