How to override existing CSS to let child <div> 1 has fixed width and child <div> has remaining width of parent?

Question

I'm trying to write Firefox stylish css (full screen width style) for our StackExchange sites.

In the tagged question list page (eg: https://stackoverflow.com/questions/tagged/java ), the HTML is like the following

<div class='question-summary'>
    <div class='statscontainer'>
        <div>n votes</div>
        <div>n answers</div>
        <div>n views</div>
    </div>
    <div class='summary'>
        <h3>Question title</h3>
        <div>question excerpt ...</div>
        <div>tag1 tag2 tagN </div>
    </div>
</div>

The original CSS use fixed width on parent/child 1/child 2

<style>
    .question-summary
    {
        float: left;
        width: 730px;
        background-color: silver;
    }
    .statscontainer
    {
        float: left;
        width: 86px;
        margin-right: 8px;
        background-color: gray;
    }
    .summary
    {
        float: left;
        width: 635px;
        background-color: lightgray;
    }
</style>

Now I try to override CSS to let it fit full screen width

    .question-summary
    {
        float: left;
        width: 100% !important;    /* parent: full screen width */
        background-color: silver;
    }
    .statscontainer
    {
        float: left;
        width: 86px;    /* child 1: fixed width */
        margin-right: 8px;
        background-color: gray;
    }
    .summary
    {
        float: left;
        /*
        width: 80% !important;   <--- how to let child 2 has remaining width ?
        left: 95px;
        right: 0px;
        */
        background-color: lightgray;
    }

The question is, how to let child 2 has remaining width ? I know when using <table> to control layout, it is pretty easy.

<table style='width:100%'>
    <tr>
        <td bgcolor='gray' style='width:80px'>fixed width</td>
        <td bgcolor='lightgray'>automatically has remaining width</td>
    <tr>
</table>

Edit

According both @sandeep and @MrLister 's answers, it should override 3 CSS properties to get this work

.question-summary .summary {
    width: auto !important;      /* original = width: 735px; */

    float: none !important;      /* original = float: left; set to 'none' to get the 'overflow' property work */
    overflow: hidden !important; /* original = not set, default is visible */
}

Answer 1

You should reset the width to its initial value, which is auto.

Edit:
As you noted though, in order for width:auto to work, you should also reset the float property, otherwise the width won't take up the rest of the available space.