Physical changes occur when the properties of a substance are retained and/or the materials can be recovered after the change. Chemical changes involve the formation of a new substance. Formation of a gas, solid, light, or heat are possible evidence of chemical change.
After one half-life, one half is remaining. After two half-lives one fourth is remaining. So that would be 25%
As overall compound is neutral so , let oxidation number of P be x
4(x) + (-2)8 = 0
x = +4
