Если еще надо
<!DOCTYPE html>
<html lang="ru">
<head>
<meta charset="utf-8">
<title>HTML-документ</title>
<script>
'use strict'
window.onload = function(){
var kolvo = document.getElementsByName('kolvo')[0];
var price = document.getElementsByName('price')[0];
kolvo.oninput = function(){
price.value = kolvo.value*10;
}
price.oninput = function(){
kolvo.value = price.value/10;
}
}
</script>
</head>
<body>
<input type="text" name="kolvo">Количество<br><input type="text" name="price" >Цена
</body>
</html>