What would the % p, on the npk ratio, be reported as for a 100g sample containing 7.5g of phosphorous?

When the concentration is expressed in percentage, you simply have to divide the amount of substance to the total amount of the mixture, then multiply it by 100. In this case, when you want to find the percentage of P in the sample, the solution is as follows:

%P = 7.5 g/100 g * 100 = 7.5%