You are required to make a 20ml 0.5M disodium hydrogen phosphate solution (Na2HPO4)?

20 mL 0.5 M Na2HPO4 is 10 mmoles. 1 mmole of Na2HPO4=2*23+1+31+3*16=46+1+31+48=126 mg/mmol, so you must dissolve 1260 mg = 1.26 g of Na2HPO4 in 20 mL to obtain a 0.5 M solution.