? Ruan Yujiang is studying for a Ph.D. in Vietnam Institute of Educational Sciences. He used a computer program to find out all possible combinations. As a result, 74 A4 sheets were printed. She said that the computer can easily work out the answer, "but the human brain is certainly not easy." One of the correct number combinations is 1, 1, 1, 1, 1, 3,7,9, 1.