You don't lose 20 lbs in 15 days, even with a gym membership and weights.
You'd have to cut your calories way too much to do that in a healthy manner. There just really is no short cut to healthy weight loss. It's one of those things that we have to work at, on an ongoing basis, not just when we have a special event or reason for losing weight.
Eating healthy foods in proper proportions and getting moderate exercise are the best ways to lose weight, but it takes time. Most likely you didn't put all the weight on overnight, likewise, you won't be able to lose it all over night.
Just do sum running, skipping or jogging around ur street or sumfin.