GED Math Test 2014 - Solve by Substitution Tutorial

You can get our 75 question GED exam with videos of all the solutions:

A math tutorial on how to solve for variables using the substitution method.

We've got two equations:
y = 8 + 4 * x
2 * y - 20 * x = 4

We've already got the first equation in the form y equals so we can plug the first equation into the y value of the second one.

2 * (8 + 4 * x) - 20 * x = 4

16 + 8 * x - 20 * x = 4
16 - 12 * x = 4
-12 * x = -12
x = 1

Now we can use x = 1 to solve for y by plugging x = 1 into either the first or second equation.

y = 8 + 4 * x
y = 8 + 4 * 1
y = 12

Solving by substitution has given us the values x = 1 and y = 12. Since the equations given were both equations of a line the point (1, 12) would be where the lines intersect.

[johnlocke]

Why wouldn't you Subtract 8x "add -8x" from negative 20x ? Im confused as to when not to use the inverse operation.