Final answer:
The buoyant force on the 3.0 kg block is 20 N.
Step-by-step explanation:
According to Archimedes' principle, the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object. In this case, the 3.0 kg block hanging from the spring scale displaces an amount of water whose weight is equal to the reading on the scale, which is 20 N. Therefore, the buoyant force on the block is also 20 N.