nikolayblinov,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
$(function(){
$('#myForm').submit(function(e){
e.preventDefault();
var input = parseInt($('#myNumber').val());
$('.my-table tbody tr').each(function(k, item){
var second = parseInt($(this).find('td:nth-child(2)').text());
$(this).find('td:nth-child(3)').text(second * input);
});
});
})
</script>
</head>
<body>
<form id="myForm"><input type="number" id="myNumber" value="5"><input type="submit" value="Do"></form>
<table class="my-table">
<thead>
<tr>
<th>Header 1</th>
<th>Header 2</th>
<th>Header 2</th>
</tr>
</thead>
<tbody>
<tr data-row-num="1">
<td>Item 1</td>
<td>10</td>
<td>10</td>
</tr>
<tr data-row-num="1">
<td>Item 2</td>
<td>100</td>
<td>10</td>
</tr>
<tr data-row-num="1">
<td>Item 3</td>
<td>1000</td>
<td>10</td>
</tr>
</tbody>
</table>
</body>
</html>