On planet X, an object weighs 5.51 N. On planet B where the magnitude of the free-fall acceleration is 1.08 g (where g = 9.8 m/s 2 is the gravitational acceleration on Earth), the object weighs 20.81 N. The acceleration of gravity is 9.8 m/s 2 . What is the mass of the object be on Earth? Answer in units of kg.