Calculating HTML input using JavaScript

Hi I am trying to calculate the percentage using the input from the HTML input

let timeSpend = document.querySelector(".time_spend")

var fSubject = (50/timeSpend)*100;
var sSubject = (25/timeSpend)*100;
var tSubject = (25/timeSpend)*100;
window.alert(fSubject);

This is my HTML input that I have the type of number but I don’t know why it still not working

<input
          type="number"
          id="text-box"
          class="time_spend"
          placeholder="How much time do you have?"
        />