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If the starting floor is +2 and we want to move 2 floors up then our target floor is

+2 + 2 = + 4 = 4th floor.
If we are at −2 floor and want to move 3 floors up then our target floor is
−2 + 3 = +1 = 1st floor.

The whole process is similar to working on the number line.
Addition of integers on a number line

On a number line, for adding a positive integer, we move forward while we move backwards for
adding a negative integer. Let us understand it with some examples.

Example 5: Add the following integers on the number line.
(a) 3 + 5 (b) 5 + (–11) (c) –2 + (–5)
Solution: (a) We start from 0 and move three steps to the right of 0 to reach 3. Then move 5 steps
further to the right as we have to add a positive number. We reach at 8.

3 steps 5 steps
1 2 3 1 2 3 4 5

–4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 10
Therefore, 3 + 5 = 8
(b) Starting from 0, we first move 5 steps to the right and then move 11 steps to the left
as we have to add a negative number. We reach at – 6.
11 steps

5 steps

–8 –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8
Therefore, 5 + (–11) = – 6.

(c) Here, we start from 0 and move 2 steps to the left to reach –2. Then, we move 5 steps
further to the left as we have to add a negative number. We reach at –7.
5 steps 2 steps

–9 –8 –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5
Therefore, –2 + (–5) = –7.

Quick Check
Identify the final position of result of each of the following w.r.t. zero (0) on a number line.

1. –12 + 6 2. 6 – 9 3. 3 + 8 4. 23 + 70

Additive Inverse

When we add two integers with the same numerical value but opposite in signs the sum is 0. These
numbers are called the additive inverse of each other.
For example, let us add 5 and (–5).

287 The Other Side of Zero