Direct and Inversely Proportion

Direct Proportion

Two variables are direct proportional if there is always a constant ratio between them.
(As one variable increases, the other also increases.)

(The direction of the arrows follows the growth of the variables.
When committing proportionality, follow the arrows direction!)

Example: more books - more money $$\downarrow \begin{array}{ccc}2 books& & 6 €\\ 10 books& & x €\end{array}\downarrow \Rightarrow 2:10=6:x\Rightarrow x=30$$

Inversely Proportion

Two variables are inversely proportional if the product of those variables is a constant.
(As one variable increases, the other decreases.)

(The direction of the arrows follows the growth of the variables.
When committing proportionality, follow the arrows direction!)

Example: more workers - less days $$\downarrow \begin{array}{ccc}2 workers& & 20 days\\ 5 workers& & x days\end{array}\uparrow \Rightarrow 2:5=x:20\Rightarrow x=8$$