The hardest puzzle

Our friend Treva gave Adam this puzzle for his birthday since she knew he loved puzzles. It is considered the hardest puzzle and though we have tried, we have not solved it. We let some friends here borrow it and they did solve it and once my kids knew that they were on a mission. Madelyn finally was the one. She solved it after about 20 minutes. Nothing like a little friendly competition to really kick in the IQ.