Answer:
[tex]F_{2} = 80\,N[/tex]
Step-by-step explanation:
Given the consideration of the inverse proportionality between force and distance, the following relationship is constructed and described below:
[tex]F_{1}\cdot r_{1}^{2} = F_{2}\cdot r_{2}^{2}[/tex]
The force when the distance is halved is:
[tex]F_{2} = F_{1}\cdot \left(\frac{r}{0.5\cdot r} \right)^{2}[/tex]
[tex]F_{2} = 4\cdot F_{1}[/tex]
[tex]F_{2} = 4\cdot (20\,N)[/tex]
[tex]F_{2} = 80\,N[/tex]