What's your body fat percentage, and are you male or female? (you're probably fine and not underweight. IMO it's hard to actually have so little fat that it causes problems)
You don't want to gain 15 pounds of fat. If you do want to gain weight, you'd want to have it be mostly muscle. (that's assuming you aren't under 6%/12% bodyfat)
Anyway, some points about muscle:
-Lift weights. Don't do much cardio since it will reduce muscle.
-You'll need to eat lots of protein and fat without a lot of carbs.
-Drink lots of water, ect.
-You'll need to eat a lot, and do this throughout the day rather than all at once.
And for fat(although it normally isn't healthy to gain lots of fat):
-Eat high-GI carbs like white bread, ect.
-Eat a lot of them.
-Try to spike your blood-sugar level by eating everything all at once.
-(which isn't healthy)
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