Nuclear/Radioactive Decay: occurs when the nucleus of an atom becomes unstable and spontaneously loses energy in the form of radiation emission.
Gamma Decay: a form of radioactive decay that a nucleus may undergo. This type of decay differs from other because no particles are ejected from the nucleus. Instead, a high energy electromagnetic radiation (gamma ray) photon is released.
The number in the box will be 225.
This example displays the form of gamma decay.