The brain develops extremely quickly in the first few years, but not all of this growth is permanent. due to its rapidity and temporary nature, this rapid brain growth is called

Due to its rapidity and temporary nature, this rapid brain growth is called transient exuberance.
This term refers to the fact that during the first two years of an infant's life, the number of dendrites in its brain grows massively and develops the brain. This slows down after this period, and then the brain is developed at a slower rate until it is fully grown.

Mass number minus atomic number
200-80=120