The following question has three statements. Study the question and the statements and decide which of the statement(s) is necessary to answer the question.

In a box containing a total of 100 balls of colors, red, blue, green and yellow, find the number of blue balls.

I) The probability of picking either a red or green ball is 13/20.

II) The probability of picking either a blue or yellow ball is 7/20.

III) The probability of picking a yellow ball is 3/20.