Concept:The ratio NPK 20:20:10 means the bag contains 10% potassium by weight.Explanation:The total weight of the fertilizer bag is 50 kg.The potassium component is 10% of the total weight.So, weight of potassium = 10%×50 kg.This is equal to 0.10×50=5.0 kg.Answer:5.0 kg, therefore the correct option is A.