To gain muscle mass reduce body fat percentage we need to to put a cut on your calorie intake or burn more calories through exercises. We need to burn more calories than what we eat. one pound of fat consists of 3500 calories .So its up to us how much we want to reduce. We need to do more of the resistance training exercises to increase muscle mass . Resistance training exercises are like crunches, push ups, reverse crunches, lunges, Squats, thigh rotation,etc. We need to do these exercises for 20 min ,4/week and do cardio activities for some 45 min so as to burn fat from our body. Cardio activities includes exercises like running, jogging, Brisk walking, etc. This type of exercise plan will help to gain weight mass but not our weight.