Book Maths Puzzle Algebra

How do you solve the following by Gaussian Elimination:
x-y+z=4
2x+4y-3z=1
5x-8y+z=2

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Book Other Question

I am trying to find out if someone made $20.00 last week and then made $45 the next week. How much did his production increase expressed in percentages? Jesus invests some money at 6%, and then invests $5,000 more than twice this amount at 9%. His total income from the two investments is $3,330. How much money did he invest at 9%?

I can figure out a regular simple interest problem, but get lost on problems like this. Help?!?!

Kathy