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1 because 1 can only be divided by itself. 1x1=1 see?
The number ' 1 ' has only one factor.
By definition, all prime numbers have exactly two factors. There is not a prime number that has only one factor.
The number 1 is not a prime number. A prime number has exactly two factors, 1 and the number itself. 1 is the only number having exactly one factor.
No single integer satisfies both requests. 80 is the only integer with exactly 5 factor pairs in that range, but it has two prime factors. 64 and 81 have one prime factor but 4 and 3 factor pairs respectively.