Astudent writes "the difference between 15 and the product of 5 and y" to describe the expression 5y-15.explain the error.

The word 'difference' indicates the use of subtraction in a Mathematical sum

The error is that the sum could be 15 - 5y instead of 5y - 15

A subtraction sum does not have the commutative properties, meaning that we cannot do the sum in any order. 

A sum 15 - 5y will give a different result with the sum 5y - 15 for the same value of y.