Auto-Double Application

Question

I am trying to create a quick program that will auto-Double the number that is entered in the text box.

My issue is that nothing happens when I enter a number in the text box.

What I want to happen is take the number that is input in the text box and automatically have the number doubled in the adjacent text box.

Question

What do I need to alter in my code so that the double of the number entered automatically populates in the second text box?

Code I tried

PHP/HTML

<body>

<form id="Form1" runat="server">

<div id="Form1" runat="server">

  <table id="table1" border="1">

    <tr>

      <td>

        <input type="text" id="txt_number1" name="txtnumber1input" value="0" maxlength="10" size="3">

      </td>

      <td>

        <input type="text" id="txt_number1_doubled" name="txtnumber1doubled" value="0" maxlength="10" size="3" readonly>

      </td>

    </tr>

    <tr>

      <td>

        <input type="text" id="txt_number2" name="txtnumber2input" value="0" maxlength="10" size="3">

      </td>

      <td>

        <input type="text" id="txt_number2_doubled" name="txtnumber2doubled" value="0" maxlength="10" size="3" readonly>

      </td>

    </tr>

  </table>

</div>

</form>

</body>

JS

     function DoMath(el) {

   var id = el.attr("id").replace("txt_", "");

   var lbltext = document.getElementById($("#txt_" + id)).innerHTML*2;

   window.alert(lbltext);

 }

 $("#txt_number1, txt_number2").on("keyup", function() {

   DoMath($(this));

 });

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Answer 1

You made some mistakes with selecting of elements

function DoMath(el) {
 id = el.attr('id')+"_doubled";
 $("#"+id).val(el.val()*2);
}

$("#txt_number1, #txt_number2").on("keyup", function() { DoMath($(this)); });

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