Different Dice

The numbers on a special dice are 1 \sqrt{1} , 2 \sqrt{2} , 3 \sqrt{3} , 4 \sqrt{4} , 5 \sqrt{5} and 6 \sqrt{6} . Each time the dice is rolled the numbers are multiplied. What is the probability of getting a rational number when the dice is rolled twice?

1 18 \frac1{18} 1 6 \frac16 0 0 2 9 \frac29

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2 solutions

Guru 2691
Oct 12, 2015

If the same numbers repeats on the second turn then the product is a rational number. There are six of these cases. The remaining cases are ( 1 , 4 \sqrt{1}, \sqrt{4} ) and ( 4 , 1 \sqrt{4}, \sqrt{1} ). Totally 8 cases. Then the probability equals 8 36 \frac{8}{36} .

Jonny Boy
Oct 21, 2015

There are 36 possible combinations.

  1. If you get the same number on the second turn, the product will be a rational number: 1 × 1 = 1 ; 2 × 2 = 2 ; . . . \sqrt{1} \times \sqrt{1} = 1 ; \sqrt{2} \times \sqrt{2} = 2 ; ... Altogether 6 combinations.

  2. The only rational numbers on the dice are: 1 \sqrt{1} and 4 \sqrt{4} . If you multiply those with the irrational numbers, the product will be irrational. Therefore the only two combinations left are: 1 × 4 = 2 × 1 = 2 \sqrt{1} \times \sqrt{4} = 2 \times 1 = 2 4 × 1 = 1 × 2 = 2 \sqrt{4} \times \sqrt{1} = 1 \times 2 = 2

A total of 8 combinations: 8 36 = 2 9 \boxed{ \frac{8}{36} = \frac{2}{9} }

Sorry for mediocre English. (English is not my native tongue :D )

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