Divide by the smallest primes, 2,3,5,7,11,etc.
If it divides without a remainder then it is a factor.
Take the quotient and continue factoring by primes.
91/2 no
91/3 no
91/5 no
91/7=13
13 is prime therefore 91=7 x 13
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91 is the only composite factor of 91. 7 and 13 are prime.
91 = 7 x 13
A person unfamiliar with prime numbers could mistake 51, 91, or any other composite number for a prime number if the person did not factor the number to make sure it had no other factors then 1 and itself.
Prime numbers have one distinct prime factor.
They are: 7 times 13 = 91