If you are going to add fuel to the main tanks, the CofG will have to move forward. So this is a forward limit problem. If you are adding weight to get to the forward limit, you must use the envelope to solve the problem..

Add the cargo and the pilot and the passengers to the given empty weight and you get 2328 kg and 568.6 index units. Plot this point and call it point A. Now add any amount of fuel [say 100 kg], to the main tanks. The new totals are 2428 kg and 586.4 index units. Plot this point and call it point B.

Now join point A to point B and note where that line crosses the forward limit of the envelope. It is very close to point B so the answer must be very close to 100 kg. Try 93 kg.

Proof: Add 93 kg to 2328 kg and 568.6 index units and you get 2421 kg and 585.1 index units which is a centre of gravity position of 2417 mm. Now find the forward limit at 2421 kg. It is 2421 - 2360 x .27 + 2400 = 2417 mm. Since the forward limit and the present centre of gravity are at the same point, 93 kg must be the correct answer.

Thanks so much for your support sir,
I sat the performance exam this morning and I got 80%.
I am very grateful.

Congratulations. That's a great result.

Bob