linear equation system

Question:
The price of 6 apples and 4 oranges is 8400 rupiahs,and the price of 2 apples and 7 oranges is 6200 rupiahs.how does the price of a apple and a orange?

The dialogue:
On january 3th 2009,07.30 pm,in Mela's home
Devi: "Mel,can you help me?"
Mela: "If i can,why not?"
Devi: "I have mathematic's question."
Mela: "Hmmm...if it difficult question,i won't answer it."
Devi: "my question about linear equation system."
Mela: "OK..I will try it."

When Mela does it,she look calm.she solved the question in 10 minutes.And she solve:
If the price of apple is x,and the price of orange is y,so we have 2 equation:
6x+4y=8400...(1)
2x+7y=6200...(2).
And then equation (1) multilpy by 1 and equation (2) multiply by 3,so we have new equation:
6x+4y=8400...(3)
6x+21y=18600...(4)
then,equation (4) minus equation (3) we get y=600.
we substitute y=600 in equation (1),we get x=1000.
So,the price of a apple is 1000 rupiahs,and the price of a orange is 600 rupiahs.

Before she does my question,she looks confuse if she can't do it.After she get the result of my question,she looks very happy.

Identity:
Name: Mela Febriyanti
Class : IX
School: SMP N 4 Magelang.

Question's category: Easy.Question for VIII of Junior High School.