Question given below is followed by three statements. Study the question and the statements. Identify which option is necessary to answer the question.

What will be the sum of two numbers?

I. Among the two numbers, the bigger number is greater than the smaller number by 6.
II. 40 % of the smaller number is equal to 30 % of the bigger number.
III. The ratio of half of the bigger number to one third of the smaller number is 2 : 1.