You analyze a sample of seawater weighting 630 grams. You find that it contains 22 grams of salt and 608 grams of H2O. What percent of seawater is salt>?

If you want to know the percentage (by mass) of a substance, you divide the mass of the substance in question (salt in this case) by the total mass of the original sample (the seawater), then multiply by 100.

%salt = 100\times\frac{\text{mass of salt}}{\text{mass of seawater}}