Losing 20 pounds in three months -- about 13 weeks -- requires a weekly weight loss of 1.54 pounds, well within the recommended goal of 1 to 2 pounds per week. To lose weight, you need to create a calorie deficit -- take in fewer calories than you expend. The fastest way to achieve this is through a combination of diet and exercise. Diet reduces your caloric intake so that your body doesn't create more fat. Exercise burns the fat you have and also revs up the metabolism so your body is more efficient at burning calories.
Do 30 to 60 minutes of moderate cardiovascular exercise three days a week over the next 13 weeks. During your workout, your exertion level should cause you to breathe harder and sweat. Choose exercises you enjoy to increase the odds that you'll stick with your training regimen. Bicycling, hiking, inline skating, aerobic classes and recreational sports are popular choices that burn fat.
Perform 20 to 30 minutes of interval training two days a week for three months. Interval training burns more calories in less time, according to "ScienceDaily." However, interval training puts stress on the body and shouldn't be performed more than two or three times a week, and not on two consecutive days. During interval training, alternate between moderate and high levels of exertion -- for example, jogging for a minute and sprinting the next minute. Intervals can be done with walking and bicycling as well.
Lift weights twice a week. Weight training builds fat-burning muscle. The more muscle you have, the more fat you burn. Using dumbbells, resistance bands or weight machines for resistance, strength train all major muscle groups in the legs, back, abs and arms.
Increase the activity in your daily life. Your workout isn't the only time to burn fat. Walk instead of ride or take the stairs instead of the escalator to burn more calories. WorldHealth.net reports that taking the stairs can decrease your risk of premature death by 15 percent. Choose activities that involve movement such as taking a walk instead of sedentary tasks such as watching TV.