Atoms And Nuclei

The nucleus ${ }_{50}^{120} X$ undergoes the series of reactions given below:

\({ }_Z^A X \xrightarrow{\alpha \text {-decay }} P \xrightarrow{\beta^{-} \text {-decay }} Q \xrightarrow{\alpha \text {-decay }} R\)

The number of neutrons in the nucleus $R$ is