my daughter is 7month old .she born with a low birth weight(1.8kg) still now her weight is 5.5kg how to gain the weight?

A. Yes it is a good weight gain your child has gained 3 1/2 kg plus in these timing which is a very good weight for the child I need not worry about it because actually what happens is it is expected for a child to gain at least 500 g in a month so if you look at the birth weight and the current weight it is very much in sync