help with mass and balance question
I'm studying for my upcoming flight performance and planning exam and I'm doing practice questions on PPLtutor and a question has come up that I just can't get my head around it.
Q: Before refuelling, an aeroplane has a weight of 1850 lb and the total moments are 153000 in.lb aft of the datum. Fuel weight equalling 520 lb with an effective arm of 9 ft aft of datum is then loaded. what is the new GC of the aeroplane.
I'd calculate as follows by making a table and the formula mass x arm = moment:
Aeroplane weight: 1850 82.70 1530000
Fuel Weight: 520 9 4680.
Total 2370 157680
New GC: 157680/2370 = 66.53.
However, this is not the answer.I looked at the solution this is what the answer was:
Aeroplane weight: 1850 82.70 1530000
Fuel Weight: 520 108 56160.
Total 2370 209160
New GC: 209160/2370 = 88.25.
Can anyone explain where the value of 108 Came from for the fuel weight ARM?