Given data:

The force acting on the box is given as,

Here, a is the acceleration of the box.
Rearranging the equation in order to get the acceleration of the box;

Substituting the known values;

The third equation of motion is given as,

Substituting the known values,
![\begin{gathered} v^2=(0)^2+2*(8.33\text{ m/s}^2)*(20\text{ m) } \\ v^2=333.2 \\ v=\sqrt[]{333.2} \\ =18.25\text{ m/s} \end{gathered}](https://img.qammunity.org/2023/formulas/physics/college/awwulyfhvadmk6rkoohg5bdu8mz9xcd55a.png)
Therefore, the velocity of box after 20 m is 18.25 m/s.