= Solution
\b[The lighter product receives the fraction $1/(1+m/M)$ of the two massive products' <kinetic energy>.] Neglecting photon <momentum>, <conservation of momentum> gives equal and opposite daughter momenta, $\mathbf p_m=-\mathbf p_M$. In the nonrelativistic approximation,
$$
K_m=\frac{p^2}{2m},\qquad K_M=\frac{p^2}{2M},\qquad\frac{K_M}{K_m}=\frac mM.
$$
Hence $K_{\rm products}=K_m(1+m/M)$ and
$$
\boxed{K_m=\frac{K_{\rm products}}{1+m/M}.}
$$
This partitions <kinetic energy> after any stipulated photon energy has been removed from the available <Q value>. It does not assign the complete mass-defect energy to the light particle.
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