Safe weight loss is 1-2 pounds/week, so if you stick with a really good diet, to lose 2 pounds/week, you could lose your 20 lbs in 10 weeks.
I have flat feet, too, but I still walk - a lot!! Google "chair exercises" and you will find free videos, to which you can exercise. Talk to your doctor about it.
If you've gained a lot of weight in a short time, you might want to have your doctor do some blood tests to make sure you don't have hypothyroidism. Weight gain is one of the main symptoms of under active thyroid.
I agree that if you drink a glass of water before you eat, you will, most likely, eat less.
Make water your drink of choice, and get plenty of lean protein and veggies to fill you up. Avoid white bread and things made with white sugar.
Drink a big glass of water or milk before you eat. I've lost 16 lbs in the last month and a half because I drink a big glass of water or milk(1%) before I eat anything. All I do for exercise is work.
I can't exercise cos i can't breath and also i have flat feet, the doctor told me to not try to walk. If u say, it's too diff to lose 20 pounds in a month. What do u think, how long i can lose in 20 pounds? Cos i never gain weight & size like this before but when i came & stay in usa ,i gain much weight & size. I can't accept and i not happy my size now.
Not to worry about the English. I understand perfectly, what you're trying to say.
You can't lose 20 pounds in one month safely. You have to make sure you provide enough food for your body to live on.
Why can't you exercise? There are exercises for nearly every capability, even chair exercises.