JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://netdna.bootstrapcdn.com/bootstrap/3.0.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap.min.css">
<div class="well">


<div class="form-group">
  <select class="form-control" id="opcao_soma_val1" name="soma">
    <option value="1" selected>1</option>
    <option value="2">2</option>
    <option value="3">3</option>
  <select>
</div>
<div class="form-group">
  <input class="form-control"  id="opcao_soma_val2" value="1" type="number" step="1" min="1" max="10">
  <div id="resultado">0</div>
  <button class="btn btn-primary" id="somar">Somar</button>
    <button class="btn btn-primary" id="subtrair">Subtrair</button>
  </div>


</div>

CSS

#resultado {
  padding: 10px;
  border: 2px solid #ccc;
  border-radius: 8px;
  margin: 20px 0;
}

JavaScript

$(function(){
  $('#somar').on('click', function(){
     somar();
  });
   $('#subtrair').on('click', function(){
     subtrair();
  });
});
function somar(){
     var val1 = $('#opcao_soma_val1').val();
     var val2 = $('#opcao_soma_val2').val();
     var result = (parseInt(val1) + parseInt(val2));
     $('#resultado').text(result);
} 
function subtrair(){
     var val1 = $('#opcao_soma_val1').val();
     var val2 = $('#opcao_soma_val2').val();
     var result = (parseInt(val1) - parseInt(val2));
     $('#resultado').text(result);
}