I have a fluid container of 96% width.

I have a box inside this container which also has percentage width.

I can't get the box to center horizontally inside the wrapper. The usual `margin:0 auto;` isn't working - I guess because it has a percentage width.

```
<div id="wrapper">
    <div class="box">
        <img class="scales"... />
    </div>
</div>

```

Here's the css:

```
#wrapper {
    width:98%;
}

.box {
    width:40%;
}

.scales {
    width:100%;
}

```