Bebras Challenge

The Bebras Challenge is a group of logic puzzles. You have 45 minutes to solve them. I think the most enjoyable one was lemonade party. I did not like it but I chose it because it was easy. You had to take the certain number of liter bottles and add them for a certain number. It was just simple addition. The most difficult one to me was crack the code. It was a nine digit answer but you only could use 2, 6, 7, 9. You had to use 9 the least and 2 the most. It was difficult because there was too many combinations for me to keep track. I don't know how to complete it to this day.