Posted by Cassie on .
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,
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,
MC 7th grade algebra.
Thanks for your help, Reiny! 
Algebra 
cc,
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)