Question 21
Nine people, numbered 1 to 9 from left to right, stand in a row. Three of them always frown, three others always smile, while the remaining three mature people sometimes frown and sometimes smile. Each neighbour of a frowner is a mature person. At a certain moment, the frowns and smiles are shown in the diagram below. Use the numbers of three mature people from left to right to form a three-digit number. What is this three-digit number?
