How do you express the quotient of 15 and y?

Is it 15 over y

yes. Generally in such a wording the dividend is mentioned first, then the divisor.

Similarly, the difference between 15 and y is

15-y

Generally the minuend is mentioned before the subtrahend. (gotcha there, didn't I?)

Yes, you can express the quotient of 15 and y as "15 over y." This is a common way to represent division or the fraction form of a division equation. The numerator, which is 15 in this case, represents the dividend or the number being divided. The denominator, which is y, represents the divisor or the number dividing the dividend. By writing it as 15/y, it indicates that 15 is being divided by y.