Posted by Cassie on Friday, February 27, 2009 at 6:01pm.
Solve the system of equations using substitution.
x + 4y = 5
3x  8y = 45
When I tried to solve it, I got x=17 and y=3. When I plugged these into the first equation, it didn't work out at all. Could someone please show me the steps so I can see where I went wrong? Thank you!

Algebra  Reiny, Friday, February 27, 2009 at 6:24pm
I doubled the first and added it to the second to get
5x = 35,
so x = 7, subbing that back I got y = 3
those values verify.

Algebra  Cassie, Friday, February 27, 2009 at 6:28pm
MC 7th grade algebra.
Thanks for your help, Reiny!

Algebra  cc, Friday, February 27, 2009 at 11:20pm
since you need to solve the system of equations by substitution, solve the first equation for x by subtracting 4y from both sides. You will get x = 5  4y.
Next substitute this expression into the x of the 2nd equation to get
3(5  4y) 8y =45
Now use the distributive property to get
15 12y  8y =45
simplify to get
15 20y = 45 (when you combine to negatives you get a negative answer)
add 15 to both sides and you get
20y = 60
divide both sides by 20 to get y = 3.
Now replace this answer into the first equation or the x = 5  4y equation to get x= 5  4(3) = 5 + 12 = 7
answer (7,3)
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