Nitrogen Fixation and Photosynthesis - Medical Science

Q1:

During nitrogen fixation the enzyme nitrogenase catalyse the reaction. The reaction is high energy demanding which require approximately

A 12 ATP

B 18 ATP

C 25 ATP

D 7 ATP

ANS:B - 18 ATP

How it needs 18 molecules, one N2 needs 8 ATP molecule to breakdown then how come it needs 18 ATP?