Algebra homework...

is that 2 and 1 half x? Have you tried using his text book?

The syntax is a little confusing but I'm interperting 3/4x - 2/5x = 2 1/2x - 3/5y = -2
as the two equations
(3/4)x-(2/5)x=-2 & (21/2)x-(3/5)y=-2
math grammer is only one verb (=) per sentance
Solving the first one by finding a common denominator (4x5=20) has the first equation as
(15/20)x-(8/20)x=-2
or
(7/20)x=-2 so x=-40/7

The second equation then becomes
(21/2)(-40/7)-(3/5)y=-2
-60-(3/5)y=-2
(3/5)y=-58
so
y=-290/3

Rap

I don't see where the -60 comes from, rap.

oh.... yes I do. You called it "21" divided by 2 and I was reading it as 2 and 1/2 or 5/2

I understand the algebra without the fractions. It's two equations.
Using both, you are supposed to isolate either x or y, and solve by using elimination.

ie..
+2x -3y = 0
-4x + 3y = -1

-2x = -1
x = 1/2
I understand this, I just don't get the fractions... it's been twenty
years. I forget. My son lives in Edmonton, I'm in Vancouver. I can't look at his text book...Sorry, I didn't write it out cleary enough. It's supposed to be:
3/4x - 2/5y = 2
1/2x - 3/5y =-2
So, if I get this correct... both equations must have a denominator of 20?


9/20x - 8/20y = 2 or 9/20x - 8/20 = 40/2
10/20x -12/20y =-2 10/20x - 12/20y = - 40/20

19/20x - 20/20y = 0 19/20x - 1 =0

19/20x = 1

x = - 19/20???
Thanks.

ohhhhhhh -- that's a completely different problem altogether! You don't need substitution. You have two equations with two unknowns. Just add the two equations to get to:

-5/4x - y = 0
-5/4x=y

Then pick either equation and solve for x:
1/2x - (3/5 * -5/4x) = -2
1/2x - 15/20x = -2
10/20x - 15/20x = -2
-5/20x = -2; divide both sides by -5/20 which means flip the fraction and multiply
x = -2(-20/5) or 40/5 or 8

then y= -5/4(8) or 10

Thank-you, thank-you, thank-you.
Now I can help him.