How do I align a <div> to the center of a <div>?

Gregory Little

I’m using HTML/CSS to make a website and on my home page I have some <div> tags that are inside <div> tags. I can’t seem to get the inner <div> tags to align to the center of the outer <div> tags. Here is the HTML code i am using (the one with the stars next to it is the one that I want to align);

    <div id="container">
<div align="center" id="box" style="background-color:black;width:375px;height:400px;border:0px solid #003300;">
<h1 id="heading"></h1>
<h2 id="otherheading"></h2>
**<div align="center" id="box" style="background-color:white;width:275px;height:225px;border:0px solid #003300;">**
<ul class="list-tick">
    <li></li>
    <hr>
    <li></li>
    <hr>
    <li></li>
    <hr>
    <li></li>
    <hr>
    <li></li>
</ul>
</div>

And this is my CSS:

#container {
  margin: auto;
  width: 1125px;
}

#container div {
  float: left;
  height: 400px;
  width: 375px;
}

Any help will be greatly appreciated. If you have further questions about the problem, I will try and explain in more detail.