just an example question:

What is the new CG if the weight is shifted from the forward to the aft compartment under loading conditions W-5?

The table provides this info for W-5:

Loaded weight = 88,300

Loaded CG(%MAC) = 25.5%

Weight change(lbs) = 3,300

Forward compartment centroid - STA 352.1 and -227.1 index arm

Aft compartment centroid - STA 724.9 and +144.9 index arm

MAC 141.5 inches, LEMAC STA 549.13, and -30.87 index arm

the answer choices are

A. +19.15 index arm

B. +13.93 index arm

C. -97.92 index arm

Since im using gleim to study they tell you the formulas and such to get the answer but i just dont understand what index arms are. The answer is A by the way.