Adding and Subtracting values with checkbox and select

Here is a simple example, since you have not made it very clear what you need.

<select id="opcao_soma_val1" name="soma">
  <option value="1" selected>1</option>
  <option value="2">2</option>
  <option value="3">3</option>
<select>

<input  id="opcao_soma_val2" value="1" type="number" step="1" min="1" max="10">
<div id="resultado"><!-- aqui vem o resultado --></div>
<button id="somar">Somar</button>
<button id="subtrair">Subtrair</button>

<script
  src="http://code.jquery.com/jquery-3.3.1.min.js"integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
  crossorigin="anonymous">
</script>
<script>

$(function(){
  $(document).on('click','#somar', function(){
     somar();
  });
 $(document).on('click','#subtrair', function(){
     subtrair();
  });

});
function subtrair(){
     var val1 = $('#opcao_soma_val1').val();
     var val2 = $('#opcao_soma_val2').val();
     var result = (parseInt(val1) - parseInt(val2));
     $('#resultado').text(result);
  } 
function somar(){
     var val1 = $('#opcao_soma_val1').val();
     var val2 = $('#opcao_soma_val2').val();
     var result = (parseInt(val1) + parseInt(val2));
     $('#resultado').text(result);
  } 
</script>

  

To sum in real time, simply blur the last field.

Example:

$('#opcao_soma_val2').on('blur', function(){
  somar();
});

Example Fiddle