var b;

var c;

var d = 0;

function setup() {

  createCanvas(400, 400);

  frameRate(60);

}

​

function draw() {

  background(0);

  stroke(255);

  line(200,200,mouseX,mouseY);

  var x = ave(200,mouseX);

  var y = ave(200,mouseY);

  var x2 = ave(200,x);

  var y2 = ave(200,y);

  var x3 = ave(x,mouseX);

  var y3 = ave(y,mouseY);

  fill(a,b,c);

  ellipse(x,y,d+30,d+30);

  fill(b,a,c);

  ellipse(x2,y2,d+30,d+30);

  fill(c,b,a);

  ellipse(x3,y3,d+30,d+30);

  a = random(0,255);

  b = random(0,255);

  c = random(0,255);

  if (keyIsDown(UP_ARROW)){

    d = d + 5;

  }

  if (keyIsDown(DOWN_ARROW)){

    d = d -5;

  }

  if (d < -30){

    d = -30;

  }

  //challenge: use the ave function again to make

  //a green dot half way between the center and the

  //red dot

  //var x2 = ??

  //var y2 = ??

  //fill(0,255,0);

  //ellipse(x2,y2,30,30);

}

function ave(num1, num2){

  return (num1 + num2) / 2; // replace with code to return average of the two arguments

}

​