How do I format the highlighted line in currency:

<script> //write the java script
var limit = parseInt(prompt("How many days?", "")); //define first variable
var total = .005; //define second variable
for(var days=1; days<=limit; days++) //define the count start
{
total = total+total; //formula for the running total
document.write("<tr>"); //output the table row
document.write("<td>"+days+"</td>"); //output first variable
document.write("<td>"+total+"</td>"); //output the second variable**************
document.write("</tr>"); //end table row
}

</script>
</table>

Do you mean style the output with CSS?

I can do that but I want to stay with just html.

I can do that but I want to stay with just html.

Let JS write a style attribute. If you can settle for HTML and CSS.
Example below.

You could write an HTML element inside the table cell. The B element might be semantically suitable in this case: