how do i gain 15 lbs. of muscle mass in a matter of months ?

I am 15, 150 lbs., very fit and eat very healthy. I work out 4 days a week. Mon., tues., thurs., fri. I take creatine, glutamine, and protein. My workouts consit of 4×4 250 lbs. bench press, 4×10 200 lbs. bench press, 4 completly alternating abb workouts, and 4×25 sit ups. What else can I do.

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