JSFiddle - React, Tailwind, and code Playground

HTML

<table>
    <tr class="selectable">
        <td class="center">
            <input type="text" name="qtd" />
        </td>
        <td class="text-center">
            <input type="text" name="valor" value="10" />
        </td>
        <td class="text-center">
            <select name="mark" id="mark" > 
                <option value="0.80">0.80</option>
                <option value="0.85">0.85</option>
                <option value="0.90">0.90</option>
                </select>
        </td>
        <td class="text-center">
            <input type="text" name="total" value="resultado" />
        </td>
    </tr>
</table>

JavaScript

function getClosest(el, tagName) {
    while (el = el.parentNode) {
        if (el.tagName.toLowerCase() == tagName) return el;
    }
}

window.addEventListener('keyup', function (e) {
    // caso nĂŁo seja o input pretendido
    if (e.target.name != 'qtd') return;
    
    // ir buscar o valor inserido
    var value = parseFloat(e.target.value);
    
    //pega o valor do select
    var selectMark = parseFloat(document.getElementById("mark").value);
    
    // ir buscar o outro valor dentro da mesma linha
    var tr = getClosest(e.target, 'tr');
    var valor = tr.querySelector('input[name="valor"]');
    
    // calcular a soma
    var mult = value * parseFloat(valor.value);
    
    //divide pelo valor
    var result = mult / selectMark;
    
    // inserir a soma no input total da mesma linha
    tr.querySelector('input[name="total"]').value = result;
});