<form oninput="totalWeight.value=parseInt(catAWeight.value)+parseInt(catBWeight.value)">0
  <input type="range" id="catAWeight" value="50">100
  +<input type="number" id="catBWeight" value="50">
  =<output name="totalWeight" for="catAWeight catBWeight"></output>
</form>
input, output {
  margin: 0px 20px;
}

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