Once, when I was 28 someone asked me when my baby was due and I suddenly realized that the baby was eight months old and I was still wearing maternity clothes. I started walking at night when the kids were in bed, for half an hour or so. I was so dumb - I'd walk my half hour and then come home and have a bowl of ice cream as a reward. Still the pounds just melted off!

So I'd say yeah, thirty minutes a day will definitely work! You can either diet OR exercise but if you do both you'll lose that 20 pounds shockingly fast. Enjoy being 18!!