Adding and Subtracting Negative Numbers | KS3 Maths

At teatime it's $4$ degrees. Overnight it falls $7$, so the temperature drops straight past zero to $4 - 7 = -3$ — three degrees below zero. By morning it's back up to $2$ degrees. How much did it rise?

A rise is the morning temperature take away the night one: $2 - (-3)$. Count the steps from $-3$ up to $2$ on the line and there are five of them:

$2 - (-3) \;=\; 2 + 3 \;=\; 5$

The two minus signs sit together and collapse into a plus — and the five you counted on the line is the same five degrees. That's the whole idea: taking away a negative is the same as adding.

Start at $5$ on the number line. Adding a negative doesn't push you up the line — it pulls you left, eight steps, straight across zero:

$5 + (-8) \;=\; 5 - 8 \;=\; -3$

So $5 + (-8)$ is exactly the same as $5 - 8$. Adding a negative is just subtracting.

You don't need the number line for this one — just spot the two signs sitting together: the subtract, and the negative right after it. They become a plus:

$-7 - (-10) \;=\; -7 + 10 \;=\; 3$

Starting at $-7$ and going up $10$ takes you past zero to positive $3$.