1212

12

HTML

<select id="inputOne2" name=tip required>
  <option value="">...</option>
  <option value="0.90">О</option>
  <option value="2.23">Ж</option>
</select><center>Количество</center>
<input type="number" min=100 id="inputTwo" name=colvo required>


   
       <center>Сумма, руб.:</center>
           
              <h4 style="margin:10px"> 0</h4>

JavaScript

var input = document.querySelectorAll('input');

var div = document.querySelector('h4');
var res;
for (let i = 0; i < input.length; i++) {
  input[i].addEventListener('input', function() {
    if (inputOne2.value !== '' && inputTwo.value !== '0') {
      res = inputOne2.value * inputTwo.value;
      div.textContent = res;
    }
  })
}