You didn't specify who you wanted the “average muscle percentage” for. Here are some averages for various adults that are of a normal weight.
Women that don't exercise, have fairly sedentary lifestyles have an average of about 24% of their weight in muscle. With a moderately active lifestyle it can go up to around 30%. High amounts of muscle, such as a weight lifter, can be as high as 35% of weight. Anything over 35% is considered extremely high, typical of competitive bodybuilders.
BMI is a measure which takes into account a person's weight and height to gauge total body fat. BMI is found by taking your weight (in kilograms) and dividing it by your height squared (in centimeters). A person with a BMI of 35 would be classified as obese.
