Question

Order 1 for 12 bushes and 15 trees that will cost the customer $600
Order 2 for 8 bushes and 20 trees that will cost the customer $680
What is the cost of one bush?

Answer 1

12b + 15t = 600 reduce by dividing by 3 giving us : 4b + 5t = 200
8b + 20t = 680 reduce by dividing by 4 giving us : 2b + 5t = 170

It is not necessary to reduce the equations to fidn the answer, it is just easier when you working with smaller numbers.
So now we have :
4b + 5t = 200
2b + 5t = 170 --->(-1)
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4b + 5t = 200
-2b - 5t = -170 (result of multiplying by -1)
------------add
2b = 30
b = 15

I know you do not need to find the trees, but I am going to just so I can check my answer.
4b + 5t = 200
4(15) + 5t = 200
60 + 5t = 200
5t = 140
t = 28

check..
2b + 5t = 170
2(15) + 5(28) = 170
30 + 140 = 170
170 = 170 (correct)

so your bushes (b) cost $ 15 a piece and your trees (t) cost $ 28 a piece