# C1_w2 practice quiz exercise 7

I’m having trouble setting up the equations for problem 7 or possibly solving them. i have

s + c + b = 10000
s - 2c = 0
.02s + .03c + .04b = 250

are my equations correct?

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Hi @Christopher_Stephens. Welcome to DeepLearning community!

Your first equation is correct. The last two equations should be derived from the last sentence of the word problem.

The second equation should tell that the sum of your money in s, c, b (where s is twice as c) are still the same (i.e. 10,000).

The third equation should be similar to the second equation in the left side but multiplied with the corresponding interest rates (0.02, 0.03, 0.04) and should equal to the total interest gained (i.e. 260, not 250).

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I’m still not seeing how to set this up.

s + c + b = 10000
2c + c + b = 10000
.02(2c) + .03(c) + .04(b) = 260

but i don’t think s should be defined in terms of c in the initial setup.

thanks for the response btw!

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They are all correct just 260 instead of 250 in last one. Matrix should be:

|1 1 1 | 10k
|1 -2 0 | 0
|.02 .03 .04 | 260

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makes perfect sense after seeing it. thanks!

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We know that s = 2c, thus we can substitute s in other 2 equations:
3c + z = 10000
.07c + .04z = 260

We eliminated s, next we can eliminate c

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