To find the mass percent composition of an element, divide the mass contribution of the element by the total mass then multiply by 100.
The total mass = 200.0 g
Phosphorus mass = 87.28 g
Oxygen mass = 112.72 g
%composition of P:
=> (87.28 g/200.0g)*100
=> 43.64 %
%composition of O:
=> (112.72g/200.0 g)*100
=> 56.36 %