How many ATP are produced from the single fatty acid chain

I got lost completely in that question, please help.

full question is : If the breakdown of one 18-carbon fatty acid produces 9 acetyl CoA molecules, how many ATP are produced from the single fatty acid chain if all the molecules complete the citric acid cycle and electron transport chain?

8.5N -7 where N = length of the fatty acid chain..One must "invest " 2 ATP ( actually hydrolyze pyrophosphate ) to get the fatty acyl CoAs into the mitochondrion for beta oxidation. Th 9 acetyl CoAs produced by sequential cutting of the fatty acid chain require two oxdations per cut ( one FAD mediated the other NAD+ mediated ) to get the chain in the form to cut off an acetyl CoA.So for a fatty acid chain of N length one gets N/2 Acetyl CoAs, (N/2)-1 FADH2 made , (N/2 ) -1 NADH produced -2 ATP to get the fatty acid into the mitochondrion...sooo N/2 acetylCoAs at 12 ATPs per acetyl CoA ( 3NADH, 1 FADH2 and 1 GTP ) 2 ATPs per FADH2, and 3 ATPs per NADH
12 ( N/2 ) = 6N, 2ATP ( N/2 -1 ) = N-2, 3ATPs ( N/2 -1 ) = 3N/2 - 3 - 2ATP for entrance into mito..combining like terms
6N + N + 3/2 N = 8.5 N -2-3-2 = -7 = 8.5 N -7 , so for an 18 carbon fatty acid
8.5 ( 18 ) - 7 = 153 -7 = 146 ATPs

It's amazing!I like this one.
http://zhun.info/312603/transport-chains