You are given the percentage of solution of phosphoric acid which is 66.6%. You are required to get the percent P in the solution. The chemical formula of phosphoric acid is H3PO4 and its molar mass is 98 grams per mole. Take a basis of 100 grams of solution so that we will have a 66.6g of H3PO4. To get the P in the solution;
66.6g H3PO4 (1 mole H3PO4/98g H3PO4)(1mole P/1mole H3PO4)(1mole P/31grams P) = 21.1 grams P
Percent P = (21.1g P/66.6g H3PO4)*100
Percent P = 31.6%