How many grams of NaCl are in a 388.5g sample of a 1.25% (m/m) NaCl solution?

The concentration given is a mass percentage of the NaCl solution, as denoted by (m/m). A concentration of 1.25% means that there are 1.25 g of NaCl solute per 100 g of NaCl solution. Thus,

388.5 g (0.0125) = 4.856 g of NaCl present