Can you play the game of Sophie and Emmy?

Sophie and Emmy play the following:

Sophie chooses an element n n from the set { 1 , 2 , 3 , . . . , 100 1, 2, 3, ... , 100 } without telling Emmy. After:

  • Emmy asks Sophie: what is the rest that is obtained by dividing n n by 11 11 ?

  • Sophie answers: Three.

  • Again Emmy asks Sophie: what is the rest that is obtained by dividing n n between p p , where p p is a prime number? Y

  • Sophie responds: Zero

With this information, Emmy was able to determine with certainty what the value of n n was.

How many different values can p p take for this situation to occur?


The answer is 4.

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1 solution

Stephen Mellor
Jan 21, 2018

From the first question, we know that n n is of the form 11 k + 3 11k+3 , meaning it is one of: ( 3 , 14 , 25 , 36 , 47 , 58 , 69 , 80 , 91 ) (3,14,25,36,47,58,69,80,91)

Writing this list in terms of its prime factors: ( 3 , 2 7 , 5 5 , 2 2 3 3 , 47 , 2 29 , 3 23 , 2 2 2 2 5 , 7 13 ) (3,2 \cdot 7,5 \cdot 5,2 \cdot 2 \cdot 3 \cdot 3,47,2 \cdot 29,3 \cdot 23,2 \cdot 2 \cdot 2 \cdot 2 \cdot 5,7 \cdot 13)

Now, our answer is the number of primes that only appear once in the above list: ( 13 , 23 , 29 , 47 ) (13,23,29,47) , so the answer is 4 \boxed{4}

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