Finally I can figure it out 
If I can put the ball by myself, I will put 5 red + 3 green in the first box and left the 1 green ball in the second box. So whenever I pick the box, I know that the box with just 1 ball on it is have the green ball.
If I can put the ball by myself, I will put 5 red + 3 green in the first box and left the 1 green ball in the second box. So whenever I pick the box, I know that the box with just 1 ball on it is have the green ball.
The answer is correct.
(4 mBTC to Jeremycoin)I'm sorry if my explanation is not that good
BTW, my username is Jeremyroll