JSFiddle - React, Tailwind, and code Playground

HTML

<table>
<tr>
    <td><input name="qty1[492]" class="qty required" type="text"></td>
    <td><input name="qty2[492]" class="qty" type="text"></td>
    <td><input name="qty3[492]" class="qty" type="text"></td>
    <td><input name="qty4[492]" class="qty" type="text"></td>
    <td><input name="qty5[492]" class="qty" type="text"></td>
</tr><tr>
    <td><input name="size1[492]" type="text" class="size required" ></td>
    <td><input name="size2[492]" type="text" class="size" ></td>
    <td><input name="size3[492]" type="text" class="size" ></td>
    <td><input name="size4[492]" type="text" class="size" ></td>
    <td><input name="size5[492]" type="text" class="size" ></td>
</tr>

</table>

JavaScript

$('.qty').keyup(function() {
        var s       = $(this).attr('name');            // = qty3[418]
        var qtyID   = s.replace(/[^1-9\[\]]/g, "");    // = 3[418] 
        var SizeID  = "size" + qtyID;
        
        var $sizeInput = $(this).closest('tr').next().find(SizeID);
        $sizeInput.css('background-color', 'green');
        $sizeInput.addClass('required'); 
        
//     $(this).parent().find(SizeID).addClass('required');                
                
                
    });