Assuming this is a triangle then the answer would be 10 but that's not an option. If you gave me more information or confirmed what shape I'm dealing with I may be able to help you.
They averaged 3lbs. a week for 4 weeks to get 12lb. They would need an additional 36lbs. to get to 48lbs. Averaging the same 3lbs. per week, this would take an additional 12 weeks for a total of 16 weeks.
