You can lose a great amount of weight during summer by engaging in exercise activities. You should also check on your diet and get a nutritionist to advice you on how to eat. However, you might not entirely lose the 20 pounds but weight loss will depend on your activeness.
I would focus more on the body fat percentage. Get physically active this summer, and look at your eating habits with a dietician/nutritionist. Try to eat as much of your food in its most natural state as you can, with your parents controlling the groceries and meals, you'll need their support. Also look at whether this is normal for you, or if its a more recent gain. That can help you figure out where the weight came from, if you weren't as heavy before. Your progress with be determined by your effort, and burning more calories than you take in every day.
make your mind agree with you, Affirm again and again that you are doing it, your weight is decreasing , Imagine what if you start loosing weight incredible fast,, Then make a goal, make a plan,make a routine,, thats the secret http://fitnessfundamentalsfree.blogspot.com/