This Law of Definite Proportion is also known as Law of Constant Composition, This law states that the relative amount of each element in a specific compound is always the same or it can also be defined as a compound always contain the same proportion by mass of its constituent element.

From the question, element A and B combines in the ratio of 9:5 which means 9 grams of A react with 5 grams of B. If 1 gram of B is reacting, then 1.8 grams of A is required which is in the ratio of 9:5. Thus defines the Law of Definite Propotion.

These results do support the law of definite proportions. We know that A combines with B in a ratio of 9:5 by mass. If we divide 9 by 5 we get 1.8, so this ratio reduces to 1.8:1. This corresponds exactly with the second observation that 1.8 g of A reacts with 1.0 g of B, this giving a mass ratio of 1.8:1. This is the exact same mass ratio as before. The law of definite proportions states that a compound always exists as set proportions of each element to each other by mass. Since A and B combine in a constant mass ratio of 1.8:1, these observations uphold the law of definite proportions.