Код

<form action="/action_page.php"

 oninput="x.value=parseInt(a.value)+parseInt(b.value)">

 0

 <input type="range" id="a" name="a" value="50">

 100 +

 <input type="number" id="b" name="b" value="50">

 =

 <output name="x" for="a b"></output>

 <br><br>

 <input type="submit">

</form>