well if you want to gain weight but not fat weight than you have to gain muscle which weight training would help. Cardio will help tone your muscles but not build alot. Now i race triathlons and have some pretty tone legs but very lean. But even cardio would probably help you add muscle weight im 5'5 and 120 pounds with 13 percent body fat. 15-25 is normal so i have very low fat and im still alot more weight so i guess cardio does help !! I would suggest changing your diet (eating more eggs, chicken, broccoli, protein) Try resistance training 2 times a week to add a little muscle but not much (also females really dont have to worry about gaining alot of muscle mass unless they are hard core since the female body really doesnt add much mass especially to the arms) With resistance training females gain strenght as quickly as males but not necessarly mass. I would continue your cardio 5 times a week. Anymore questions ask im an exercise science major studying to be a personal trainer and i been an triathlete for 3 years. so i have some experience in the field of fitness.