P5.js code for creating a calculator tip app confused

p5.js Coding Questions
31 
var pickEvent
var img;
var img2;
var img3;
var sel;
var sel1;
var title;
var bill;
var answer;
var output;

// preload some images
function preload() {
  img = loadImage('mandarin.jpg');
  img2 = loadImage('bostonpizza.jpg');
  img3 = loadImage('nandos.jpg');
}

function setup() {

title = createElement('h1', 'Tip calculator'); // Create the main title for the question
title.position(265, 40); // Set the position of the title

  
  createCanvas(900, 500);  

  bill = createInput(); // Create the input box for the user to enter a number
  bill.position(312, 175); // Set the position of the input box

  textAlign(CENTER);
  background(200);
  sel = createSelect();
  sel.position(315, 110);
  sel.option('Mandarins');
  sel.option('Nandos');
  sel.option('Boston Pizza');
  sel.selected(pickEvent);
  

  textAlign(CENTER);
  background(200);
  sel1 = createSelect();
  sel1.position(325, 245);
  sel1.option('Very Bad');
  sel1.option('Bad');
  sel1.option('Okay');
  sel1.option('Good');
  sel1.option('Great');
  sel1.option('Excellent');
  sel1.option('Random Amount');
  sel1.selected('Service');
  output = createElement('h3', "") // Creating a placeholder for the final output

button = createButton('tip it'); // Create a button and the name inside the button
button.position(345, 285); // Set the position of the button
button.mousePressed(calculation); // When the mouse button is pressed on the button it goes to the calculation function

}

function draw() {

background(153);

let pickEvent = sel.value()

  if (pickEvent == 'Mandarins') {
    background(img);
  } 
  if (pickEvent == 'Nandos') {
    background(img3);
  } 
  if (pickEvent == 'Boston Pizza') {
    background(img2);
  }

  // Calculate the conversion
function calculation() {

let conversion = sel1.value(); // Set the drop down menu option to the variable conversion

//convert fahrenheit to celsius
if (sel1 == ('Very Bad')) { // Run this if the selection is set to F to C
answer = round(((bill.value() * .01), 2)) // Math to convert
total = answer + bill.value()
output.html('This is the answer: ' + answer + '\u2103') // Create the screen output for the user. \u2103 is the HTML CSS degrees C symbol
output.position(315, 500) // Set the position of the answer

}
}
}