JSFiddle - React, Tailwind, and code Playground

HTML

<form action="" method="POST">
      <input type="text" name="occurences" id="occurences" value="" />
      <input type="text" name="recurringAmt" id="recurringAmt" value="" />
      <input type="text" name="totalAmt" id="totalAmt" value="" />
    </form>

JavaScript

/* On page contents loaded */
    function updateForm() {
      var occ = $('#occurences');
      var occV = parseFloat(occ.val());
      occV = occV >= 0 ? occV : 0;
      var rec = $('#recurringAmt');
      var recV = parseFloat(rec.val());
      recV = recV >= 0 ? recV : 0;
      var tot = $('#totalAmt');
      var totV = parseFloat(tot.val());
      totV = totV >= 0 ? totV : 0;
      
      /* If total is disabled */
      if (tot.attr("disabled")) {
          if (rec.val() == '') { /* if no text in rec */
            tot.removeAttr("disabled"); /* Reenable total */
            tot.val('');
            return;
          }
          /* Otherwise update total */
          tot.val(recV * occV);
          return;
      }

      /* If rec is disabled */
      if (rec.attr("disabled")) {
          if (tot.val() == '') { /* if no text in total */
            rec.removeAttr("disabled"); /* Reenable rec */
            rec.val('');
            return;
          }
          /* Otherwise update rec watching for divide by zero error */
          rec.val(occV > 0 ? totV / occV : 0);
          return;
      }

      /* Otherwise neither disabled */

      if (recV > 0) { /* if rec has a number value */
          tot.attr("disabled", true); /* disable total */
          tot.val(recV * occV); /* update total */
          return;
      }
 
      
      if (totV > 0) { /* if total has a number value */
          rec.attr("disabled", true); /* disable rec */  
          /* Update rec watching for divide by zero error */
          rec.val(occV > 0 ? totV / occV : 0); 
          return;
      }
    }

    $(document).ready(function() { 
      $('#occurences').keyup(function(){ 
        updateForm();
      }); 
      $('#totalAmt').keyup(function(){ 
        updateForm();
      }); 
      $('#recurringAmt').keyup(function(){ 
        updateForm();
      }); 
    });