# Math Help - Why this last step when changing the subject?

1. ## Why this last step when changing the subject?

Hi, I'm really pleased to say that I have learned lots from your answers, thank you again. I do not understand why the last step is taken in the question below. (Change the subject of the formula to y)

$x-2y=10$

$x-10=2y$

$2y=x-10$

$y=\frac{x-10}{2}$

$y=\frac{x-5}{2}$

I understand that they must have used the common denominator. I thought when you divide one part by the common denominator, you had to divide the other part also, so how do they know that 2 will go into x?

Marilyn

2. Hello Marilyn
Originally Posted by Meggomumsie
Hi, I'm really pleased to say that I have learned lots from your answers, thank you again. I do not understand why the last step is taken in the question below. (Change the subject of the formula to y)

$x-2y=10$

$x-10=2y$

$2y=x-10$

$y=\frac{x-10}{2}$

$y=\frac{x-5}{2}$

I understand that they must have used the common denominator. I thought when you divide one part by the common denominator, you had to divide the other part also, so how do they know that 2 will go into x?

Marilyn
You're right to query it - the last step is incorrect! The answer is

$y=\frac{x-10}{2}$

and you can't simplify it any further!