2019 AIME II Problems/Problem 4
A standard six-sided fair die is rolled four times. The probability that the product of all four numbers rolled is a perfect square is , where and are relatively prime positive integers. Find .
Notice that, other than the number 5, the remaining numbers 1, 2, 3, 4, 6 are only divisible by 2 and/or 3. We can do some cases on the number of 5's rolled (note that there are outcomes).
Case 1 (easy): Four 5's are rolled. This has probability of occurring.
Case 2: Two 5's are rolled.
Case 3: No 5's are rolled.
To find the number of outcomes for the latter two cases, we will use recursion. Consider a 5-sided die with faces numbered 1, 2, 3, 4, 6. For , let equal the number of outcomes after rolling the die times, with the property that the product is a square. Thus, as 1 and 4 are the only possibilities.
To find given (where ), we observe that if the first rolls multiply to a perfect square, then the last roll must be 1 or 4. This gives outcomes. Otherwise, the first rolls do not multiply to a perfect square ( outcomes). In this case, we claim that the last roll is uniquely determined (either 2, 3, or 6). If the product of the first rolls is where and are not both even, then we observe that if and are both odd, then the last roll must be 6; if only is odd, the last roll must be 2, and if only is odd, the last roll must be 3. Thus, we have outcomes in this case, and .
Computing , , gives , , and . Thus for Case 3, there are 157 outcomes. For case 2, we multiply by to distribute the two 5's among four rolls. Thus the probability is
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