You have a stock solution of 50 mg/L PO43-“as P”. The stock solution is what you use to prepare your standards (0 ,5, 10, 20, 30 mg/L PO43-“as P”). How man milliliters of stock solution do you need to prepare your 30 mg/L PO43-“as P” standard in a 100 ml volumetric flask? Use C1V1=C2V2