.contenedor {
    padding: 20px;
    width: 80%;
    margin:auto;
    font-size:25px;
    background: #2c3e50;
    color: #fff;
  }
  .calc{
    margin-left:20%;
    border:3px solid #fff;
    margin-right:20%;
    padding:20px;
    
  }
  .boton{
    height:40px;
    width:60px;
    text-align:center;
    vertical-align:top;
    font-size:15px;
    }