HTML: при нажатии на переключатель radio - отобразить числовое поле ввода в той же линии

#own:not(:checked) ~ #own-size { display: none } 
#own-input {display: flex} 
#own-input label {flex-basis: 100%; }

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous"> 
 
 
<h3>Размер</h3> 
 
<div class="d-flex flex-column align-items-start"> 
 
  <div class="custom-control custom-radio"> 
    <input type="radio" class="custom-control-input" name="answer" checked="checked" value="no" / id="man"> <label class="custom-control-label" for="man"> <h5>Стандартный мужской</h5> </label> 
  </div> 
  <div class="custom-control custom-radio"> 
    <input type="radio" class="custom-control-input" name="answer" value="yes" id="woman" /><label class="custom-control-label" for="woman"> <h5>Стандартный женский</h5> </label> 
  </div> 
  <div class="custom-control custom-radio text-left mb-4" id="own-input"> 
    <input type="radio" class="custom-control-input" name="answer" value="other" id="own" /> <label class="custom-control-label" for="own"> <h5> Указать размер</h5> </label> 
 
    <div class="input-group mb-2 mr-sm-2" id="own-size"> 
      <label class="sr-only" for="product-own-size-input">Size</label> 
      <input type="number" class="form-control" id="product-own-size-input" placeholder="размер"> 
      <div class="input-group-append"> 
        <span class="input-group-text">см</span> 
      </div> 
    </div> 
  </div> 
 
 
</div> 
 
 
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