To find the fraction by which the amount of chlorine decreased:

Let *x* be the amount of chlorine in the pool yesterday.

The amount of chlorine in the pool today is `5/18` of *x*, or `5/18*(x)`

The amount of decrease = amount of chlorine yesterday - amount of chlorine today.

`x- 5/18 x`

By making the denominators the same and subtracting, we get

`(18x-5x)/18` `rArr (13x)/18`

which can be written as `(13/18) x` and means `13/18` of *x*.

The quantity *x* has been decreased to `5/18` of *x* by subtracting `13/18` of *x*.

So the fraction by which the amount of chlorine decreased is **`13/18` **.