The law of conservation of mass states that mass can neither be created nor destroyed. For example in an isolated system, mass of the reactants is equal to the mass of the products. Thus, mass or the quantity of the system under consideration will remain unchanged through any physical or chemical process.
In the given example, the board initially has a mass of 750 g. It is physical transformed to sawdust by grinding. Based on the conservation of mass, the pile of sawdust would still weigh 750 g.
