Object b is at rest under the action of gravity and tension, so F=GB. Through the force analysis of the pulley, the pulley is subjected to the upward tension of the spring, its own gravity and the tension of the ropes at both ends. According to the balance of forces, we can know that the spring scale indicates =G pulley +2F, so the gravity of object B is GB =12n;

So the answer is: 12.