What is the probability of drawing a ball that is not blue?

• A. 1/3
• B. 1/2
• C. 2/3
• D. 4/5

Answer: (B)

Probability here hinges on the idea of a complementary outcome: not blue is everything that isn’t blue, so its probability is the complement of drawing blue.

If blue balls make up half of the total, then not-blue also makes up the other half. That gives a probability of drawing a not-blue ball as 1/2. A simple example: four balls total with two blue and two not blue. Drawing one at random then yields a not-blue result half the time.

In general, you’d compute P(not blue) as (number of not-blue balls) ÷ (total number of balls). If that ratio is 1/2, the probability is 1/2.