The first thing to do is to work out the actual c of g positions.
The length of the MAC is 200, TE - LE, 1000 - 800.
10% of 200 is 20.
So the original c of g is 20 rearward of the MAC LE.
Therefore the actual position is 20+800 = 820.
25% of 200 is 50, so the position of the new c of g is 850, using the same method.
The way that I work out such calculations is to use the following method. It is not what is tought where I am studying, but it is more logical to me because I am not learning a formula, I am actually understanding what is going on.
(New moment of c of g) = (Old moment of c of g) - (moment of where mass is being taken) + (moment of where mass is being added)
Mass moved is unknown.
(Mass X new c of g) = (Mass X old c of g) - (Mass moved X arm of position mass to be taken out of) + (Mass moved X arm of position mass to be added to)
(70000 X 850) = (70000 X 820) - (Mass moved X 400) + (Mass moved X 1200)
59500000 = 57400000 - (400 X Mass moved) + (1200 X Mass moved)
2100000 = 800 X Mass moved
Mass moved = 2625kg
Edited since I forgot to include the mass unit kg.
Hope this helps,
PA28.