A75.00 g a sample of a substance is analyzed and found to consist of 32.73 g phosphorus and 42.7 g oxygen . calculate the percent composition of this substance.

The sum of the parts in this problem is slightly bigger than the total
32.73 + 42.7 = 75.43 
Then the percentage composition is not going to be 100% exact but you can calculate some approximation in the following way

%= Part x 100/Total  

phosphorous Part = 32.73 g
oxygen  Part = 42.7 g
Total = 75.00 g

% phosphorous  = 32.73 g x 100 / 75.00 g = 43.64% phosphorous  

% oxygen  = 42.7 g x 100 / 75.00 g = 56.93 % oxygen  

What does that mean you dum bum