In any fission process the ratio

$\frac{\mathrm{mass} \mathrm{of} \mathrm{fission} \mathrm{products}}{\mathrm{mass} \mathrm{of} \mathrm{parent} \mathrm{nucleus} } \mathrm{is}$

In fission process, when a parent nuclei breaks into daughter products, then some mass is lost in the form of energy. Thus, mass fission products < mass of parent nucleus.

$\Rightarrow \frac{\mathrm{mass} \mathrm{of} \mathrm{fission} \mathrm{products}}{\mathrm{mass} \mathrm{of} \mathrm{parent} \mathrm{nucleus} } <1$