radioactivity Questions

A radioactive nucleus ${}_Z^{A}X$ undergoes spontaneous decay in the sequence:
$${}_Z^{A}X\stackrel{1}{\to }{}_{Z-1}B\stackrel{2}{\to }{}_{Z-3}C\stackrel{3}{\to }{}_{Z-2}D$$
The possible decay particles in the sequence are:

A nucleus with mass number $240$ breaks into two fragments each of mass number $120$. The binding energy per nucleon of unfragmented nuclei is $7.6\text{MeV}$ while that of fragments is $8.5\text{MeV}$. The total gain in the Binding Energy in the process is:

A half-life of a radioactive nuclide is $100$ hours. The fraction of original activity that will remain after $150$ hours would be: