Question 1076553
40x+3y+9z=35
15x+16y+11z=50.5
11x+10y+300z=370 
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Inconvenient coefficiencts.
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I would use determinants, or Cramer's rule.
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For the Det:
<pre>
|40    3    9|
|15   16   11|
|11   10  300|
</pre>
Det = 40*(16*300 - 10*11) - 3*(15*300 - 11*11) + 9*(15*10 - 11*16)
Det = 187600 - 13137 - 234
Det = 174229
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Do you know how to do the rest?
You can check your answers using an online matrix solver.