How to calculate the mass percent of an unknown phosphate given concentration?

Solution A has 330.6 mg/L. So in 250 mL it has (330.6 / 4) 82.65 mg. This is how much you dissolved in 250 mL. 83.65 mg = 0.08365 grams. So the mass percentage is 0.08365 / 0.1137 *100 = 73.57%