Maths riddle ~ help plz!

A + B + C = 7
C = 2 * (A + B) - 5
A + 3 * B + 2 * C = 15

substitute C in the first and last equation
A + B + 2A + 2B - 5 = 7
A + 3B + 4A + 4B - 10 = 15

You get
3A + 3B = 12
5A + 7B = 25

A + B = 4
A = 4 - B

20 - 5B + 7B = 25
2B = 5
B = 2.5

A = 4 - B
A = 1.5

C = 7 - A - B
C = 3

check:
A + B + C = 7 :
1.5 + 2.5 + 3 = 7? yes.
C = 2 * (A + B) - 5 :
3 = 2 * (1.5 + 2.5) -5? yes.
A + 3 * B + 2 * C = 15:
1.5 + 3 * 2.5 + 2 * 3 = 15? yes.

Answer + working given

Another way
Man i worked for 1 1\2 hours on this 1! lol! I found it easier just to work in Microsoft excel... here is how

1.) The A-C coloumn is where you put all the numbers to add up to 7
2.) in the D column you type =SUM(A37:C37) and then copy and paste how ever many of those workspaces you want. (when i say paste it means highlight the bar right under the one u copied and drag down until u have as many as u want highlighted and then right click and select paste.)
3.) in the E column it will give you the answer to 2(x+y)-5 (column c and column e should match to get it right) just type =SUM(2*(A22+B22))-5 in the first bar then paste however many of those you want in your workspace.
4.)in the F column will be the answer to x+3y+2z. OPf course to get hte problem right it will have to equal 15. just type =SUM(A3+(3*B3)+(2*C3)) in the first bar and follow the paste instructions above.

Now you can have as many slots as you want to use the guess and check method....it may sound a bit complicated but it really isnt.

exampple of what it might look like:

a= answer except for the A naming the column!

A B C D E F

x y z 7 A A

Uhm, 1 1/2 hours? Isn't that a bit long for a three equation set?

maybe but i kept screwing up....and i tried diferent ways until i thought of the excel thing....SOME of us may not be as quick as OTHERS

maybe but i kept screwing up....and i tried diferent ways until i thought of the excel thing....SOME of us may not be as quick as OTHERS

lol, I didn't mean it that way...

I meant that there is a standard set of rules you can follow, so you should be done in a relatively short time. If you want, I'll post the rules I learned.