Display numbers in a order using for loop in JavaScript

I tried this code and got the output below:

<script type="text/javascript">

document.write("<table border='1' width='70%'>");
    for(var a=1;a<=20;a=a+5){
      document.write('<tr>');
      for(var b=a;b<a+5;b++){

          document.write("<td>"+b+"</td>");

      }
      document.write('</tr>');
    }
    document.write("</table>");

    </script>

Output:

 1  2  3  4  5
 6  7  8  9 10
11 12 13 14 15
16 17 18 19 20

How can I display the output like this:

 1  6 11 16
 2  7 12 17
 3  8 13 18
 4  9 14 19
 5 10 15 20