hi 40 kgs at 20 is quite less, it can also be affected by genetic factors, just make sure that they is no underlying cause like diabetes, thyroid etc., take some good protein diet with fruits and vegetables and do some regular exercise

Hello,     Weigh houldbe according to your height and nor according to your age.You have.not mentioned ypur hoight so how can I  calculate your BMI. BMI= Weight in kg divided by Height in metre square   Should be between 19- 25 If your BMI is less than 19 you are considered to be underweight. To gain weight eat good amount of protein,less amount of carbohydrates and fats.Include nuts,fruits,leafy vegetables,milk in ypur diet.Eat in time Never skip meals.Have a heavy healthy breakfast.. Exercise for at least half an hour daily and later 15 min of warm ups.   Never go to bed late nights.Maintain atleast 1 to 11/2 hour gap between dinner and bedtime.. Early to bed and early to rise will improve your weight. Check reasons for not gainig weight like thyroid,blood sugars,Vit D3,any worm infestations.etc.and get them rectified. Wish you a healthy living.