"20" isn't the correct answer, because you could have managed it with two converted tries and two penalties. "21" isnīt it either, for three converted tries suffice, and "22" also doesnīt work, for you can manage it with less than 4 tries.
NB. The scoring is: 3 for a drop-goal or penalty kick. 5 for a try, and 7 for a converted try (A converted try is when a try has been scored, and the team that scored kicks the ball between the posts.)