Hint: One possible solution is to have a sprite that starts at the left side of the stage and moves slowly towards the right. Every time the user clicks on "b" for "back" the sprite is moved to the back. Hitting the "b" key is detected in the Stage script which broadcasts a message "moveBack". The moving sprite listens for this message, and when it recieves it it: a) moves itself back to x location zero; and b) increments the global count. When the timer gets to zero the game stops by removing the sprite.
Note: there is no reason to follow this hint, this is just a suggestion if you are stuck.