Question 190608
Let 

x = # of trees that cost $12


y = # of trees that cost $8



Since he bought 12 trees total, this means that {{{x+y=12}}}



Also, because "Redwood trees cost$12 each and spruce trees cost$8" and the total came to $104, this tells us that {{{12x+8y=104}}}



So we have the system:



{{{system(x+y=12,12x+8y=104)}}}



{{{x+y=12}}} Start with the first equation.



{{{y=12-x}}} Subtract {{{x}}} from both sides.



{{{y=-x+12}}} Rearrange the terms.



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{{{12x+8(-x+12)=104}}} Now plug in {{{y=-x+12}}} into the second equation.



{{{12x-8x+96=104}}} Distribute.



{{{4x+96=104}}} Combine like terms on the left side.



{{{4x=104-96}}} Subtract {{{96}}} from both sides.



{{{4x=8}}} Combine like terms on the right side.



{{{x=(8)/(4)}}} Divide both sides by {{{4}}} to isolate {{{x}}}.



{{{x=2}}} Reduce.



So the ranger bought 2 trees that cost $12 a piece.



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Since we know that {{{x=2}}}, we can use this to find {{{y}}}.



{{{y=-x+12}}}  Go back to the first isolated equation.



{{{y=-2+12}}} Plug in {{{x=2}}}.



{{{y=10}}} Combine like terms.



So the ranger bought 10 trees that cost $8 a piece.