Two elements A and B combine to form a compound in which ‘a’ gram of A combines with 'b1' and 'b2' gm of B. According to law of multiple proportion,

Mass of element A is fixed i.e., a gram.Masses of element B are b1 and b2 According to law of multiple proportions, masses of element that combine with fixed mass of another element must bear a simple whole number ratio. Therefore, b1 and b2 bear a simple whole number ratio.