How do you determine whether the number is a perfect square: 15?

A perfect square is a number that is the result you get from squaring a number. 15 is not a perfect square.

Explanation:

A perfect square is a number that is the result you get from squaring a number. For example, 25 is a perfect square because:

${5}^{2} = 25$

Another way to say this is that the square root of a perfect square is a whole number:

sqrt("perfect square")="whole number"

25 meets this test:

$\sqrt{25} = 5$

So with that known, we can safely say that the number 15 is not a perfect square - it isn't the result of a squaring operation. We can also look at the square root of 15:

$\sqrt{15} \cong 3.873$