# How do you subtract -3.1 - 21.9?

Jun 27, 2018

Mostly likely: $- 3.1 = - 21.9 = \textcolor{b l u e}{- 25.0}$
...but see below

#### Explanation:

Normal evaluation is from left to right;
so $- 3.1 - 21.9$
would commonly be evaluated as
$\textcolor{w h i t e}{\text{XXX}} \left(- 3.1\right) - \left(21.9\right)$
$\textcolor{w h i t e}{\text{XXX")=color(white)("xx")-3.1color(white)("xxx}} 3.1$ units to the left of zero on the number line
$\textcolor{w h i t e}{\text{XXX"=)-ul(color(white)("x")21.9color(white)("xxx}}$move $21.9$ units further to the left
$\textcolor{w h i t e}{\text{XXX")=color(white)("x")-25.0color(white)("xx}}$ now a total of $25$ units to the left of zero.

$\underline{\text{However}}$
Sometimes evaluation is done right to left:
So $- 3.1 - 21.9$
would be evaluated as $- \left(3.1 - 21.9\right) = - \left(- 17.2\right) = + 17.2$