The number of calories burned by doing 20 jumping jacks can vary depending on various factors such as weight, intensity, and duration. On average, a person weighing around 150 pounds can burn approximately 3-4 calories per minute of moderate intensity jumping jacks. Assuming it takes around 1 minute to perform 20 jumping jacks, you could estimate burning about 3-4 calories by doing them. However, this is just an approximation and the actual number of calories burned may differ for each individual.