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I have this: A person threw a standard dice three times. He obtained two distinct odd prime numbers in two throws and an even number which is not a factor of 18 in the third throw. The sum of all the numbers on the opposite faces of numbers obtained in the three rows is? ( Answer : 9 )

In this, It is given, He obtained two distinct odd prime numbers in two throws Distinct odd prime numbers in a dice are 3 i.e., 1,3 and 5

And an even number which is not a factor of 18 in the third throw So the number will be 4.

But how do I find the sum of all numbers on the opposite faces of numbers in three rows? A supportive explaination would do great:)

yena shah
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  • Trivia fun fact: The opposite sides of a die add to seven. $1$ is opposite $6$, $2$ is opposite $5$ and $3$ is opposite $4$. There's no math... just ... trivia. – fleablood Oct 22 '18 at 01:24
  • So if the three rolls are $a + b + c$ then the opposite sides will be $(7-a) + (7-b) + (7-c) = 21 - (a+b+c)$. – fleablood Oct 22 '18 at 01:26

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The sum of a face with its opposite face is always $7$. So we have that the opposite faces of $3$, $5$ and $4$ are $4$, $2$ and $3$ respectively. From there we have the answer: $4+2+3=9$.

White Crow
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    So yes, I made a mistake considering 1 as a prime number and thus was unable to figure that out. And I forgot the fact that 1 is neither prime nor composite =_= Anyways, thanks! – yena shah Oct 22 '18 at 08:49