From the given, we have 232 90Th. Thorium has 90 protons in its nucleus, the number of neutrons is calculated by subtracting 90 from 232 which gives an answer of 142. Then, for the second given 228 88Ra, Radium has 88 protons and the number of its neutrons is calculated by subtracting 88 from 228 which is equal to 140.
The final equation that would give us the proper nuclear decay reaction,
232 90Th --> 228 88Ra + 4 2a
where 4 2a is an alpha particle.
