Question

A box contains 7 yellow, 5 blue, and 4 red balls. Three balls are drawn at random. Find the probability that:

a)
All three balls are the same color
c)
Two balls are yellow and one is blue
d)
All three balls are different colors

Answer 1

a) 7/80 = 0.0875

b) 3/16 = 0.1875

c) 1/4 = 0.2500

Explanation:

There are a total of 16 balls. 3 are selected. There are ₁₆C₃ possible combinations.

a) If all 3 balls are the same color, they are either all yellow OR all blue OR all red.

The number of ways of choosing 3 yellow balls from 7 is ₇C₃.

The number of ways of choosing 3 blue balls from 5 is ₅C₃.

The number of ways of choosing 3 red balls from 4 is ₄C₃.

The total probability is:

P = (₇C₃ + ₅C₃ + ₄C₃) / ₁₆C₃

P = (35 + 10 + 4) / 560

P = 49 / 560

P = 7 / 80

P = 0.0875

b) The number of ways of choosing 2 yellow balls from 7 is ₇C₂.

The number of ways of choosing 1 blue ball from 5 is ₅C₁.

The probability of both is:

P = ₇C₂ ₅C₁ / ₁₆C₃

P = 21 × 5 / 560

P = 105 / 560

P = 3 / 16

P = 0.1875

c) If all 3 balls are different colors, then there is exactly 1 yellow, 1 blue, and 1 red.

The number of ways of choosing 1 yellow ball from 7 is ₇C₁.

The number of ways of choosing 1 blue ball from 5 is ₅C₁.

The number of ways of choosing 1 red ball from 4 is ₄C₁.

The probability of all events is:

P = ₇C₁ ₅C₁ ₄C₁ / ₁₆C₃

P = 7 × 5 × 4 / 560

P = 140 / 560

P = 1 / 4

P = 0.2500