Losing 20 pounds in 2 weeks is totally unhealthy. Just cut the junk, exercise and eat right! July 4th is in 2 weeks and it is only safe to lose about 2-3 pounds every week. So, if we multiply 2*3 = 6 pounds lost or 2*4 = 8 pounds lost during that time period. That is the only really safe amount of weight you can lose. Maybe you should explain this to your parents and ask them to extend it to August 4th or something. Good luck!