Expert: Bobby Soltani - 12/30/2004

Question
I have a couple problems that i could really use some help with. You have to solve these two problems out step by step by using the addition or subtraction method.

1. 2x + y = 18
  2x - y = 22

2. 12a - 5b = 18
  15a - 5b = 30

Answer
Hi Jermaine,

To use the addition or subtraction method, you want one of the variables to cancel.  For the first problem we can use the addition method without any modification to either equation since the y's will drop out.


2x + y = 18
2x - y = 22
add them
4x = 40
divide both sides by 4
x = 10
plug x=10 back into the first equation
2*10 + y = 18
subtract 20 from both sides
y = -2

For the second problem, we can multiply the first equation by -1 so that the b terms will cancel when we add the equations.  Try it out and let me know if you have any questions.

Bobby