title: Drawing concentric squares
body: <p>When going through the book "How to think like...", I got stuck with exercise 4.9.2.</p>
  
  <p>The question is: "Write a program to draw this. Assume the innermost square is 20 units per side, and each successive square is 20 units bigger, per side, than the one inside it" </p>
  
  <p><a href="https://i.stack.imgur.com/5FD70.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5FD70.png" alt="enter image description here"></a></p>
  
  <p>The following code represents how far I got till now:</p>
  
  <pre><code>import turtle
  wn = turtle.Screen()
  wn.bgcolor("lightgreen")
  tess = turtle.Turtle()
  
  def draw_square(t,size):
      for i in range(4):
          tess.forward(size)
          tess.left(90)
  
  size = 20
  for j in range(3):
     tess.pensize(3)
     draw_square(tess,size)
     size = size + 20
     tess.penup()
     tess.goto(-20, -20)
     tess.pendown()
  
  wn.mainloop()
  </code></pre>
  
  <p>Can someone be so kind and show me the right direction please?</p>
  
  <p>Thanks!</p>
  
  <p>Swen</p>