Answer
See explanation.
Work Step by Step
If the mass number of 2 radionuclides are given by $4n +k$ and $4n' +k$, then the heavier one can decay into the lighter one by a series of $\alpha$ and $\beta$ decays. This is because radioisotopes doesn't decay directly into a stable state, but rather it forms chain until reaching the most stable isotope. All of the decay products belong to the same family.
The mass number, A of a radionuclide changes by 4 in an $\alpha$ decay and is unchanged in a $\beta $ decay.
If $A = 4n + k$, then after $\alpha$ decay with time (m), its mass number becomes
$A = 4n + k – 4m = 4(n – m) + k$
