Solve for x: x + 1/2x = 4 ...

When simplifying
x + (1/2)x = 4
2 is left out for some reason.

You should treat both sides of the equation the same way.
For example, multiply both sides by 2:

2x + 2*(1/2)x = 2*4
2x + x = 8
3x = 8
x = 8/3