Final answer:
During hand washing, hands must be rubbed for at least 20 seconds.
Step-by-step explanation:
During hand washing, hands must be rubbed for at least 20 seconds. According to the CDC, it is recommended to hum the 'Happy Birthday' song from beginning to end twice, as it usually takes about 20 seconds. Rubbing your hands vigorously and making sure to cover all areas, including the thumbs, wrists, areas between the fingers, and under the fingernails, helps to remove dirt and microorganisms effectively.