CTET 2 Math and Science Sep 2015 Paper

The formal operational stage begins at about age 11. As students enter this stage, they gain the ability to think in an abstract manner, the ability to combine and classify items in a more sophisticated way, and the capacity for higher-order reasoning.
At about age 11+ years, the child begins to manipulate ideas in its head, without any dependence on concrete manipulation; it has entered the formal operational stage. He/she can do mathematical calculations, think creatively, use abstract reasoning, and imagine the outcome of particular actions