"20" isn't the correct answer, because you could have managed it with two converted tries and two penalties. "21" isn´t it either, for three converted tries suffice, and "22" also doesn´t 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.)