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show, or simply imagine, the scale of the number line and the positions of numbers on
it

Example, this unmarked number line (UNL) shows the addition problem: 85 + (– 60) =
?

We then can visualise that 85 + (– 60) = 25
The following UNL shows a subtraction problem which can also be written as a missing
addend problem: (–100)–(+250) = ?
or 250 + ? = –100.

We can then visualise that ? = –350 in this problem.

 Converting subtraction to addition and addition to subtraction

Target Floor – Starting Floor = Movement needed
or
Target Floor = Starting Floor + Movement needed

If we start at 2 and wish to go to –3, what is the movement needed? First
method: Looking at the number line, we see we need to move –5 (i.e., 5 in the
backward direction). Therefore, – 3 – 2 = – 5. The movement needed is –5.
Second method: Break the journey from 2 to –3 into two parts. a. From 2 to 0,