जीन पियाजे के अनुसार, बालकों की सोच अमूर्तता की अपेक्षा मूर्त अनुभवों एवं प्रत्ययों से किस आयु अवधि में होती है?

According to Jean Piaget's theory of Cognitive development, at which stage can most children 'operate on operations' and develop hypothetico-deductive thinking?

A child understands that adding four to five yields nine, but cannot yet perform the reverse operation of taking four from nine. According to Jean Piaget, this limitation indicates which stage of cognitive development?