What will be the mass of products if 10 g of substance A and 15 g of substance B combines?

As per the Law of Conservation of Mass, matter cannot be created nor destroyed in a chemical reaction. So the mass of products remains the same as reactants in a chemical reaction. 

Mass of Reactants = Mass of products.
Here in the given question, we are provided with two reactants, that is- Reactant A and Reactant B with 10g and 15g mass, respectively. Using  the Law of Conservation of Mass, we can write-

Mass of substance A + Mass of substance B = Mass of product
So,
Mass of product will be = 10g + 15g

Mass of product = 25g

Therefore, we can say that 10 and 15 g of reactants combine to give 25g of the product.