JSFiddle - React, Tailwind, and code Playground
HTML
<input id="input" type=text> of 100
<button role="button" id="go">Go</button>
<div id="result">
JavaScript
function quintile (value, population)
{
var q = Math.floor((value / population) * 5) + 1;
return Math.min( Math.max( q, 1), 5);
}
$("#go").on("click", function() {
$("#result").text(
quintile ( $("#input").val(), 100 )
);
});