You can't put a time limit on weight -- it's dependent on too many factors that make individuals very different. You don't say why you want to gain weight or why it has to be in 20 days, but gaining weight is pretty easy if health isn't of any concern to you -- eat more and more and more until your body starts putting on fat. It gets harder if you want to put on lean weight, because then you have to keep eating in a healthy way, but if all you want is to weigh more, eat more.
I'm opposite and trying to go the other way. Sigh. Which of us is better off, ha ha. What about increasing what you eat in a healthy way? Adding in some good for you fats and increasing your portion size a bit more. Lift some weights too, that can add a bit of weight to your frame as well. Why do you have just 20 days to gain weight?
Eat more food than you usually eat before as this is the easiest way to gain a couple of pounds. Never skip meals. Eat a lot of protein rich food, starchy food, eg. beans, pulses, eggs, fish, and meat and also potato, dairy products, almonds, low-fat cheese, and yogurt. Hit the gym three times at least. Avoid fats like red meat, margarine, etc. as they are bad for the health of your heart.
Hey thank you, special mom, I am "need to gain" type person but I am gaining my weight in healthy terms not in a unhealthy terms.