Heart beat causes the flow of blood round the body. This heart beat can be felt as pulse in the wrist or neck carotid artery. The heart rate which is measured in beats per minute (BPM) is used to determine the number of heart beats per minute.
You can calculate your BPM using the carotid artery found in the neck close to the windpipe.
Given that for 20 seconds, Bill had a total of 24 beats.
60 seconds = 1 minute.
Hence, Bill's BPM = (24 beats per 20 seconds) * (60 seconds per minute) = 72 beats per minute