The relative formula mass (Mr) of a compound is found by adding up the relative atomic masses (Ar) of all the atoms in its formula. For water, H2O, the sum is 1 + 1 + 16 = 18. For calcium carbonate, CaCO3, it is 40 + 12 + (3 × 16) = 100.
If a formula has brackets, multiply everything inside the bracket by the number outside it. In Ca(OH)2 the sum is 40 + 2 × (16 + 1) = 74.
The percentage by mass of an element in a compound shows how much of the total mass is that element. Work out the mass of the element in one formula (Ar × number of atoms), divide by the relative formula mass of the compound, then multiply by 100. In magnesium oxide, MgO, the relative formula mass is 24 + 16 = 40, so magnesium makes up 24 ÷ 40 × 100 = 60% of it by mass. In carbon dioxide, CO2, the relative formula mass is 12 + (2 × 16) = 44, so carbon makes up 12 ÷ 44 × 100 = 27.3% by mass.