Moles

Right, this is a long question because it's in different parts, I get parts a) and b) but I'm stuck on c) ... Please helppp, the answer for c) is 3.2 g but I don't really understand why:
4) A scientist burnt 300 g of metal X in an unsealed beaker .
She then weighed the contents of the beaker and found that it now
weighed 500 g. All of metal X reacted to form a single product, a
metal oxide.
a) Explain why the mass increased
b) Analysis of the metal oxide shows that contains 40% oxygen by mass.
What is the percentage mass of metal X in the oxide?
c) The scientist plans to use the metal oxide as part of a mixture.
The final mixture contains 24% metal X by mass, which all comes from
the metal oxide. What mass of the metal oxide was used to make 8.0 g
of the mixture?

You should make some attempt to answer the questions yourself first ...

Right, this is a long question because it's in different parts, I get parts a) and b) but I'm stuck on c) ... Please helppp, the answer for c) is 3.2 g but I don't really understand why:
4) A scientist burnt 300 g of metal X in an unsealed beaker .
She then weighed the contents of the beaker and found that it now
weighed 500 g. All of metal X reacted to form a single product, a
metal oxide.
a) Explain why the mass increased
b) Analysis of the metal oxide shows that contains 40% oxygen by mass.
What is the percentage mass of metal X in the oxide?
c) The scientist plans to use the metal oxide as part of a mixture.
The final mixture contains 24% metal X by mass, which all comes from
the metal oxide. What mass of the metal oxide was used to make 8.0 g
of the mixture?

A) The mass increased because the metal reacted with the oxygen when it was burnt as the beaker was not sealed

B) if 40% of the metal oxide is oxygen then 60% must be metal

C)
24%/60%=0.4 (Find the s.f of percentage that the metal has decreased by)

0.4*40%=16% (Find the new percentage of oxygen using the s.f)

16%+24%=40% (add the percentages to find the total percentage of metal oxide by adding both new percentages for each)

0.4*8 =3.2g (Find 40% of the 8g of the mixture)

Hope this helps!!