I need help adding a Discount checkbox/checkboxes to my Javascript Price Calculator [closed]

i have found this code for a js calculator here on this site and it is great! Javascript Price Calculator Issues (Checkboxes) Now want to add a checkbox/radio button to calculate a discount (e.g. if checked 25% off)

And if thats possible: i would love to have a pulldown menu for one category (all with the same price...lets say 5) so if i select 3 its 15 and so on.

Any help would be greatly appreciated!

//ARRAY
var eventEquipmentArray = new Array();
eventEquipmentArray["15 Inch Speakers"] = 5;
eventEquipmentArray["18 Inch Subwoofer"] = 10;
eventEquipmentArray["LED Par Cans"] = 5;
eventEquipmentArray["Smoke Machine"] = 5;
eventEquipmentArray["Moving Head"] = 10;
eventEquipmentArray["4 Gun Laser System"] = 5;
eventEquipmentArray["Red Gun Laser System"] = 5;
eventEquipmentArray["1500W Strobes"] = 10;
eventEquipmentArray["Mirror LED Lighting"] = 5;

//CHECKBOX - EVENT EQUIPMENT
function getEventEquipment() {
  var EventEquipment = 0;
  var theForm = document.forms["quote"];
  var selectedEquipment = theForm.elements["selectedEquipment"];

  for (var i = 0; i < selectedEquipment.length; i++) {
    if(selectedEquipment[i].checked){
        EventEquipment += eventEquipmentArray[selectedEquipment[i].value] || 0;
    }
  }

  return EventEquipment;
}

//DIV - TOTAL PRICE TEST
function getTotals() {
  //var totalPrice = getEventDuration() + getEventSuburb() + getEventEquipment();
  var totalPrice = getEventEquipment();
  var totalPriceDIV = document.getElementById("totalPrice");
  totalPriceDIV.innerText = "Total: $ " + totalPrice;
}

<form id="quote">
  <p>
    <label>
      <input type="checkbox" name="selectedEquipment" value="15 Inch Speakers" id="selectedEquipment_0" onChange="getTotals()" /> 15" Speakers</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="18 Inch Subwoofer" id="selectedEquipment_1" onChange="getTotals()" /> 18" Subwoofer</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="LED Par Cans" id="selectedEquipment_2" onChange="getTotals()" /> LED Par Cans</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="Smoke Machine" id="selectedEquipment_3" onChange="getTotals()" /> Smoke Machine</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="250W Moving Head" id="selectedEquipment_4" onChange="getTotals()" /> 250W Moving Head</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="Mirror LED Lighting" id="selectedEquipment_5" onChange="getTotals()" /> Mirror LED Lights</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="4 Gun Laser System" id="selectedEquipment_6" onChange="getTotals()" /> 4 Gun Laser Light</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="Red Gun Laser System" id="selectedEquipment_7" onChange="getTotals()" /> Red Laser Star Light</label>
    <br />
    <label>
      <input type="checkbox" name="selectedEquipment" value="1500W Strobes" id="selectedEquipment_8" onChange="getTotals()" /> 1500W Strobes</label>
    <br />
  </p>

  <div id="totalPrice" style="color: red; text-align: center; font-size: 18px;"></div>

</form>