Winnie wrote the following riddle i am a number between 60 and 100 my ones digit is two less than my tens digits im a prime number. winnies friend guessed that her riddle was about 79. why cant 79 be the answer.

The answer is that 79 IS NOT correct because the riddle says "my ones digit is 2 LESS than my tens digit.." and 79 has 7 in tens and 9 in ones which means the ones digit is 2 MORE than the tens digit.

To solve 
x has to be more than 60 and less than 100
the primes between those are 61,67,71,73,79,83, 89, 97
97 is two less in the ones than in the tens so it completes all of the requirements of the riddles therefore, 97 is the answer.