3 Dice Rolls

A fair die is rolled three times. The probability that you get a larger number each time is a b \frac { a }{ b } , where a a and b b are coprime positive integers. What is a+b?


The answer is 59.

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1 solution

Julian Uy
Dec 15, 2014

When a die is rolled 3 times, there are 6 · 6 · 6 = 216 possible outcomes. If a 1 is rolled first, then there are 10 possible outcomes for the next two digits such that the third digit is greater than the second, which is greater than the first. If a 2 is rolled first, then there are 6 possible outcomes for the next two digits where the digits increase. If a 3 is rolled first , there are 3 ways for the digits to increase, and if a 4 is rolled first, there is 1 way for digits to increase. So there are a total of 10 + 6 + 3 + 1 = 20 total outcomes. Thus the probability is 20 216 = 5 54 \frac { 20 }{ 216 } =\frac { 5 }{ 54 } .
5 + 54 = 59 5+54=\boxed { 59 } .

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