One way would be to add absolute positioning to both divs.
or
Other way would be to use table. For example:
<table>
<tr><td>
<form method="post">
<div style="float:right;display:inline;">Show Me:
<select id="ddNumRecords" name="ddNumRecords" onchange="this.form.submit();">
<option>30</option>
<option>50</option>
<option>100</option>
</select>
</div>
</form>
</td></tr>
<tr><td>
<div>
<div style="float:left;
min-height:1px;
padding:15px 2% 20px;
position:relative;
width:96%;">
hello world
</div>
</td></tr>
</table>
or
Add one more div around the div that has float:right. For example:
<form method="post">
<div style="float:left; width:300px">
<div style="float:right;display:inline;">Show Me:
<select id="ddNumRecords" name="ddNumRecords" onchange="this.form.submit();">
<option>30</option>
<option>50</option>
<option>100</option>
</select>
</div>
</div>
<div>
<div style="float:left;
min-height:1px;
padding:15px 2% 20px;
position:relative;
width:96%;">
hello world
</div>