In one example 10 grams element A combines with 5 grams and 15 grams B in two compounds. What is the mass ratio of B?

Correct answer: 1:3

The direct answer is 1:3, so option B is correct. The question compares the masses of B that combine with the same mass of A, namely 10 grams. In the first compound, 10 grams of A combine with 5 grams of B. In the second, the same 10 grams of A combine with 15 grams of B. Therefore the mass ratio of B in the two compounds is 5:15. Divide both terms by their common factor 5: 5 ÷ 5 = 1 and 15 ÷ 5 = 3. Thus the simplest whole-number ratio is 1:3. Option A, 1:2, would represent 5:10, not 5:15. Option B, 1:3, is correct because it is the simplified form of 5:15. Option C, 2:3, does not simplify the given numbers. Option D, 3:5, reverses or changes the required comparison and is not equal to 5:15. This illustrates the law of multiple proportions: when the same masses of one element combine with different masses of another, those different masses are in a small whole-number ratio. The exam method is to write the two masses in order and divide both by their greatest common divisor.