Final answer:
To factor the expression 15x - 20xy correctly, you need to factor out the greatest common factor from both terms.
Step-by-step explanation:
The expression 15x - 20xy is incorrectly factored by your friend as 5x(3 - 4xy). To factor the expression correctly, we need to factor out the greatest common factor from both terms. In this case, the greatest common factor is 5x. By factoring out 5x, we get:
15x - 20xy = 5x(3 - 4y)
So the correct factored expression is 5x(3 - 4y).