= Solution
At step $n$, the parent and daughter rest masses are
$$
m(R_n)=nM+m_0,
\qquad
m(Q)+m(R_{n-1})=m+(n-1)M+m_0.
$$
The decay is allowed when $M\geq m$, which holds because $M=\lambda m$ with $\lambda>1$. Hence all $N$ stages occur:
$$
R_N\longrightarrow R_0+NQ.
$$
The initial rest mass is $NM+m_0$ and the final rest mass is $Nm+m_0$. Thus
$$
\boxed{\Delta m=N(M-m)=Nm(\lambda-1)}
$$
of rest mass is converted into <kinetic energy>, corresponding to energy $Nm(\lambda-1)c^2$.
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