Yes, it is possible to lose 20 pounds in 5 months. You will have to stick to a diet and excercise regime in order for you to get results. If you have someone that can do it with you will help keep you motivated or a personal trainer.
I did it the wrong way, but I lost 50 pounds in a month by running and walking 3 times a day for ten minutes each and hardly ate a thing. My fiance on the other hand lost 100 pounds in less than 6 months by only drinking water and eating bananas. We are now working out at the gym 5 days a week and i have lost 22 pounds in a week and a half by working out, mostly cardio (running), drinking lots of water, and only eating fish, grilled chicken, fruits and veggi soup. Hope this helped.