eat high protine content, protine build muscle. eat a lot of banana and other protine content such as soyabean, pulses, egg. do some muscular excersie e.g weight training(dont over do it). dont try to increase ur diet, have healthy diet.
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why do u want to gain weight. I was of same size as u in 2005. now i have put on weight slightly more than what u want. (70kgs). I having a tought time, as people comment on ur weight gain. Now really want to loose weight. So i would suggest better dont go for weight gain.