Answer: There is no set weight for any age, as it depends upon the height and body type of the individual. This can range from 60 to 120 pounds (27 to 54 kg) for 11-year-old girls, with a median weight of 82 pounds (37 kg).
Weight itself is also not the important factor. The ratio of muscle to fat is an important determinant in overall health. To increase or decrease weight, diet and exercise are the controlling factors in young people.
(see the related question for information and links)
Answer: I cant say that you would loose a total of 30 lbs in a matter of 4 weeks, but low-carb diets, when done correctly provide a healthy weight loss. Typically the first week people loose 5-10 lbs from water weight and then slowly loose from there, depending on what your intake is like. If you arent sure how to do a low carb diet, Atkins is a more extreme form of it and can give you and idea of what your body is doing to loose that weight.
Answer: This looks like a typo...If you mean "how much does gas weigh", automobile gasoline weighs approximately 6 lbs. per US gallon. I leave it to you to do the necessary conversions to Imperial gallons, litres, kilograms, etc.