Pickin' Five

A bag contains:

  • 1 r e d {\color{#D61F06} red} ball
  • 2 g r e e n {\color{#20A900} green} balls
  • 3 y e l l o w {\color{#EC7300} yellow} balls
  • 4 b l u e {\color{#3D99F6} blue} balls

You reach in and pull out five at random.

What is the probability that you pull out the following:

  • 1 r e d {\color{#D61F06} red} ball
  • 1 g r e e n {\color{#20A900} green} ball
  • 1 y e l l o w {\color{#EC7300} yellow} ball
  • 2 b l u e {\color{#3D99F6} blue} balls

If this probability is a b \dfrac{a}{b} where a a and b b are coprime positive integers, what is a + b a+b ?


The answer is 8.

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1 solution

Geoff Pilling
Jun 26, 2018

There are ( 10 5 ) = 252 \binom{10}{5} = 252 ways of choosing 5 balls.

And, if you choose 1 red, 1 green, 1 yellow and 2 blues, there is:

  • 1 1 way to choose the red ball
  • 2 2 ways to choose the green ball
  • 3 3 ways to choose the yellow ball
  • ( 4 2 ) = 6 \binom{4}{2} = 6 ways to choose the blue balls

That is 1 2 3 6 = 36 1 \cdot 2 \cdot 3 \cdot 6 = 36 ways.

So, the probability of choosing this combination is 36 252 = 1 7 \dfrac{36}{252} = \dfrac{1}{7}

1 + 7 = 8 1+7 = \boxed8

( 10 5 ) = 252 \binom{10}{5}=252 not 36.The probability is 36 252 \frac{36}{252}

X X - 2 years, 11 months ago

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Ooops, you are right... Thanks for catching the typo!

Geoff Pilling - 2 years, 11 months ago

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