Losing weight in summer is a must for every girl, if you are not very fat. There is no need to lose 20 pounds at once. This cardinal number is too big to lose. Losing so much at once is definitely bad for your health. You can start to stay up late at night without dinner, slowly add some exercise to yourself, and go out for a run if you have the conditions. If there are no conditions, skipping rope or climbing stairs at home can lose at least a few pounds a month. I wish you an early success in losing weight.