AS Chemistry query

Here's the question: A passenger jet contains 4050 kg of copper wiring.
A rock sample contains 1.25% CuFeS2 by mass.
Calculate the mass, in tonnes, of rock needed to produce enough copper wire for
a passenger jet. (1 tonne = 1000 kg)

The previous part of the question (6.1) asked to balance the equations:
4CuFeS2 + 9/2 O2 + 4SiO2 → Cu2S + Cu2O + 7SO2 + 4FeSiO3
Cu2S + 2Cu2O → 6Cu + SO2

My thought process was, calc moles of Cu in 4050kg, which is 4,050,000/63.5= 63,779....

Then in the balanced equations 6 moles of Cu originally comes from 4 moles CuFeS2, so multiply these moles by 4/6, to get 42,519.... moles CuFeS2. Calculate the mass, this would be molesxMr=42,519...*183.5
=7,802,362.205g =7802.362205kg.
In the rock, 1.25% = CuFeS2, so multiply by 100/1.25, then divide by 100 to get answer in tonnes.

I got: 624.1889764 tonnes. The correct answer was 936 tonnes, and they used the original 63,779... moles to calculate the mass of CuFeS2, I don't understand why and if anyone could help out that would be greatly appreciated.

“Then in the balanced equations 6 moles of Cu originally comes from 4 moles CuFeS2” - this does not obey the law of conservation of mass, as it implies that 2 moles of copper atoms appeared from nowhere. This is a big hint that you’ve used an incorrect ratio.

I don’t think you’ve balanced the first equation correctly - firstly, look at the number of oxygen atoms on either side and secondly, try a different initial number of moles of CuFeS2.

I balanced the first equation differently and so, I got an equation wherein the right-hand side had 1 mole of Cu2S and 2 moles of Cu2O on it, among the other products.

With the first equation as I balanced it, I followed essentially the same method as you outlined and reached the same answer as the MS.

OHHHHHHHH thank you very much, no clue how I didn't spot this one to be honest.

Appreciate it!