Ira S. answered 10/22/14
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You have 810 units of copper. 3 units are used in transistors, 3 in resistors and 2 in chips. So,
3t + 3r + 2c = 810
You have 419 units of zinc, 1 for transistors, 2 for resistors and 1 for chips. So,
t + 2r + c = 419
Likewise for glass
2t + 2r + 2c = 490
That last equation makes this easy to solve. If you double the second equation and then subtract the third equation, you can solve for r.
2t + 4r + 2c = 838
2t + 2r + 2c = 490 subrtract to get
2r = 348
r = 174
Go back and substitute this in and finish the problem.
Hope this helped