Three 3 sided die

You roll three 3-sided fair dies, each numbered 1 to 3.

If the probability of having the sum of the three being an odd number is a b \dfrac{a}{b} , where a a and b b are coprime positive integers , what is a + b a+b ?


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The answer is 41.

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2 solutions

Ujjwal Rane
Aug 29, 2016

The three dies are generating 27 permutations, which can be considered as 27 consecutive numbers of a ternary number system. Since the first one 111 (equivalent to decimal 9+3+1 = 13) is an odd number, so will be the last, yielding 14 odd numbers (sums). This makes the probability of a odd sum = 14/27

Ah, nice approach, @Ujjwal Rane !

Geoff Pilling - 4 years, 9 months ago

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Thanks! Non decimal base ~ and mix base ~numbers (different base for each digit) hold a lot of untapped potential :-)

Ujjwal Rane - 4 years, 9 months ago

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@Ujjwal Rane Your comment inspired me to write this problem:

Base what?

Thanks for the inspiration! :0)

Geoff

Geoff Pilling - 4 years, 9 months ago
Geoff Pilling
Aug 27, 2016

If you enumerate them, you'll find there are 14 14 ways to roll an odd number, and 27 27 total ways to roll the die. So the probability of rolling an odd number is 14 27 \frac{14}{27}

14 + 27 = 41 14+27 = \boxed{41}

I got 26/27 as the probability. I did 1-( probability of no odds) .

Ashish Sacheti - 4 years, 9 months ago

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Sorry, I had worded the problem badly. I've updated the wording.

Geoff Pilling - 4 years, 9 months ago

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No worries it's okay!

Ashish Sacheti - 4 years, 9 months ago

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