JSFiddle - React, Tailwind, and code Playground

HTML

<div id="amount">200</div>
<div class="form-group"><label for="up_Cuotas">Cuotas </label><select id="up_Cuotas" class="form-control" name="up_Cuotas" onchange="calculate(this.value)">
<option value="2 Cuotas">2 Cuotas</option>
<option value="3 Cuotas">3 Cuotas</option>
<option value="4 Cuotas">4 Cuotas</option>
<option value="5 Cuotas">5 Cuotas</option>
<option value="6 Cuotas">6 Cuotas</option>
<option value="7 Cuotas">7 Cuotas</option>
<option value="8 Cuotas">8 Cuotas</option>
<option value="20 Cuotas">20 Cuotas</option>
</select></div>
<p> </p>
<div><label for="up_Pagos">Monto a Pagar en Cuotas </label> <input id="up_Pagos" name="up_Pagos" type="text" /></div>

JavaScript

var a = parseFloat('200.66');
var numberPattern = /\d+/g;
var pagos = document.getElementById("up_Pagos");

function calculate(meth){
	var c, up_Pagos;
    var b = meth.match(numberPattern)[0];
    c = a/b;
    up_Pagos = a/b*1000;
    pagos.value = up_Pagos.toFixed(2);

//document.up_Cuotas.up_Pagos.value = c;
}