Max earns $4$ euros per hour.
The number of hours that Max works varies.
If he works $\blue 3$ hours a day, he earns $4 \times \blue3 =12$ euro.
On another day, he works $\blue 5$ hours and earns $4 \times \blue 5=20$ euro.

In math, a varying quantity is represented by a letter, such as $\blue x$.
Now assume Max works $\blue x$ hours.
Max earns $4$ euro per hour; hence, he earns $4\times \blue x$ euro in total.
If Max works $\blue 7$ hours, then $\blue x = \blue 7$ and he will earn $4 \times \blue 7 =28$ euro in total.

With $\blue 3,\blue5$ or $\blue 7$ hours of work, Max earns:

$$\begin{array}{rcl} 4 \times \blue3 &=& 12 \\ 4 \times \blue 5 &=& 20 \\ 4 \times \blue 7 &=& 28 \end{array}$$
With $\blue x$ hours of work, Max earns:
$$\begin{array}{c} 4 \times \blue x \end{array}$$