JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.aspnetcdn.com/ajax/jquery.validate/1.13.1/jquery.validate.js"></script>
<form id="myform">
    <input type="text" name="num_gals" placeholder="Num of Gallons" />
    <br />
    <input type="checkbox" value="TRUE" name="full_tank" />Full Tank?
    <br />
    <input type="submit" />
</form>

JavaScript

$(document).ready(function () {

    $('#myform').validate({
        rules: {
            num_gals: {
                required: function () {
                    return !($('input[name="full_tank"]').is(':checked'));
                }
            },
            full_tank: {
                required: function () {
                    return ($('input[name="num_gals"]').is(':blank'));
                }
            }
        },
        messages: {
            num_gals: {
                required: 'Provide Num Gals or Check Full Tank'
            },
            full_tank: {
                required: 'Provide Num Gals or Check Full Tank'
            }
        },
        submitHandler: function(form) { // for demo
            alert('valid form');
            return false;
        }
    });

});