If Y has exactly 2 positive integer factors then Y is a prime number. if so how?

Step-by-step explanation:

yes, if your teacher meant to include the default factors of every positive integer number : 1 and the number itself.

these are the 2 positive integer factors.

if a positive number can only be divided without remainder by 1 or by itself (with result 1), then it is per definition a prime number.

if the number can be divided without remainder by another number, then we are not dealing with a prime number.

but if your teacher did not have these default factors in mind, then no :

example : 15

15 has only 2 factors : 3 and 5.

both are positive integer numbers.

and yet 15 is not a prime number.

but back to my first point, 15 also has formally the factors 1 and 15. so, if we count them too, then we have suddenly 4 factors. and the original statement fits again.

step-by-step explanation: