The balancing numbers in an equation are the ratio of the moles of each substance. So if you know the masses of the reactants and products, you can work out the balanced equation. Convert each mass in grams to moles using moles = mass ÷ relative formula mass. Then divide every number of moles by the smallest number of moles. Finally, if needed, multiply to get a simple whole number ratio.
Example: 4.8 g of magnesium reacts with 3.2 g of oxygen (O2) to make 8.0 g of magnesium oxide. Magnesium has an Ar of 24, so 4.8 ÷ 24 = 0.2 mol. Oxygen has an Mr of 32, so 3.2 ÷ 32 = 0.1 mol. Magnesium oxide has an Mr of 40, so 8.0 ÷ 40 = 0.2 mol.
Dividing by the smallest, 0.1, gives a ratio of 2 : 1 : 2 for magnesium : oxygen : magnesium oxide. These are the balancing numbers, so the equation is 2Mg + O2 → 2MgO.