JSFiddle - React, Tailwind, and code Playground

HTML

<table>
    <tr class="selectable">
        <td class="center">
            <input type="text" name="qtd" id="qtd" />
        </td>
        <td class="text-center">
            <input type="text" id="valor" 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" id="total" value="resultado" />
        </td>
    </tr>
</table>

JavaScript

//inicializa as variaveis
var inputQtd = 0;
var inputValor = 0;
var inputMark = 0;

function updateValue(){
	//atualiza os valores
    inputQtd = parseFloat(document.getElementById("qtd").value);
    inputValor = parseFloat(document.getElementById("valor").value);
    inputMark = parseFloat(document.getElementById("mark").value);
    
    //atualiza o valor no resultado
    var total = document.getElementById("total");
    
    total.value = (inputQtd * inputValor ) / inputMark;
}

document.getElementById("mark").addEventListener('change',function(e){
	updateValue();
});

window.addEventListener('keyup', function (e) {
    // caso não seja o input pretendido
    if (e.target.name != 'qtd') return;
    updateValue();
});