Two blocks can slide freely on a frictionless table. The blocks are made of a very hard, non-sticky material. Block A has a speed of 3.0 m/s and is moving toward block B, which is initially at rest. The blocks collide. After the collision, block A has a velocity v_a = 1.0 m/s. If block A has a mass of 2.0 kg, what is the mass in kg of block B? Use any reasonable assumption about the collision that allows you to solve for the mass.