Can walking every other day for 30 minutes help you lose 20 lbs over summer?

No, you need to have a sufficient amount of exercise. Exercising only moderately for three months every other day will not help you lose that much weight in such little time. You need to exercise at least six times a week. Doing some kind of cardio for 30 to 45 minutes and strength training will help you achieve your goal easier and in a shorter amount of time.