Combining like terms, step by step

Collecting like terms is tidying up: the x parts go together, the plain numbers go together, and nothing else moves.

The question

Collect the like terms.

2x + 4x + 20

2 + 4=6

20=20

6x+20

There are two kinds of thing on this line and they cannot mix: the x terms, and the plain numbers. Sort them into two piles before adding anything — the x terms are 2x and 4x, and the numbers are 20.
Add up the x pile, keeping the signs: 2 + 4 = 6, so 6x.
There is only one plain number, so that pile is already done: 20.
So it is 6x + 20 — and that is finished, even though it is still two things. Squashing them into 26x would be answering "6 whats and 20 whats?", and there is no answer to that until somebody says what x is.

How it works.

01
There are two kinds of thing on this line and they cannot mix: the x terms, and the plain numbers. Sort them into two piles before adding anything — the x terms are 2x and 4x, and the numbers are 20.
02
Add up the x pile, keeping the signs: 2 + 4 = 6, so 6x.
03
There is only one plain number, so that pile is already done: 20.
04
So it is 6x + 20 — and that is finished, even though it is still two things. Squashing them into 26x would be answering "6 whats and 20 whats?", and there is no answer to that until somebody says what x is.