A bag has 3 black, 4 white balls. Probability of picking 2 white balls?

Explanation

• Choosing 2 white balls from the 4 available gives C(4,2) = 6 ways.
• Choosing any 2 balls from all 7 gives C(7,2) = 21 ways.
• So the probability is 6/21, which simplifies to 2/7.
• Order does not matter when two balls are drawn together, so combinations are used instead of permutations.
• The step-by-step method agrees: (4/7) × (3/6) = 12/42 = 2/7, because one white ball is gone before the second draw.