Sophie and Emmy play the following:
Sophie chooses an element from the set { } without telling Emmy. After:
Emmy asks Sophie: what is the rest that is obtained by dividing by ?
Sophie answers: Three.
Again Emmy asks Sophie: what is the rest that is obtained by dividing between , where is a prime number? Y
Sophie responds: Zero
With this information, Emmy was able to determine with certainty what the value of was.
How many different values can take for this situation to occur?
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From the first question, we know that n is of the form 1 1 k + 3 , meaning it is one of: ( 3 , 1 4 , 2 5 , 3 6 , 4 7 , 5 8 , 6 9 , 8 0 , 9 1 )
Writing this list in terms of its prime factors: ( 3 , 2 ⋅ 7 , 5 ⋅ 5 , 2 ⋅ 2 ⋅ 3 ⋅ 3 , 4 7 , 2 ⋅ 2 9 , 3 ⋅ 2 3 , 2 ⋅ 2 ⋅ 2 ⋅ 2 ⋅ 5 , 7 ⋅ 1 3 )
Now, our answer is the number of primes that only appear once in the above list: ( 1 3 , 2 3 , 2 9 , 4 7 ) , so the answer is 4