The answer is the formal operational adolescent systematically tested alternative hypotheses.
The pendulum problem is a test designed to measure a child or adolescent's level of cognitive development. Research has indicated that children in the concrete operational stage approached the pendulum problem by randomly and sloppily trying out different variations. On the other hand, adolescents approached the pendulum problem by systematically testing each combination or hypotheses, indicating that they had achieved the formal operational stage of cognitive development.