Answer:
Explanation:
Force between two charges can be expressed as follows 
F = k q₁ q₂ / d²
q₁ and q₂ are two charges , d is distance between them , k is a constant whose value is 9 x 10⁹
distance between charges is fixed which is 4 -2 = 2 cm = 2 x 10⁻² m 
force between 1μC and  4μC
 = 9 x 10⁹ x 1 x 4 x 10⁻¹² / ( 2 x 10⁻² )²
= 9 x 10 = 90 N 
 force between 4μC and  1μC
 = 9 x 10⁹ x 4 x 1 x 10⁻¹² / ( 2 x 10⁻² )²
= 9 x 10 = 90 N 
 force between 2μC and  2μC
= 9 x 10⁹ x 2 x 2 x 10⁻¹² / ( 2 x 10⁻² )²
= 9 x 10 = 90 N 
force between 1μC and  2μC
= 9 x 10⁹ x 1 x 2 x 10⁻¹² / ( 2 x 10⁻² )²
= 4.5 x 10 = 45 N 
force between 1μC and  8μC
= 9 x 10⁹ x 1 x 8 x 10⁻¹² / ( 2 x 10⁻² )²
= 18 x 10 = 180 N 
force between 2μC and  8μC
= 9 x 10⁹ x 1 x 8 x 10⁻¹² / ( 2 x 10⁻² )²
= 36 x 10 = 360 N 
Left Charge   Right Charge Resulting force(N)
1μC                     4μC                  90 N
4μC                   1μC                    90 N
2μC                  2μC                    90 N
1μC                    2μC                   45 N
1μC                  8μC                    180 N
2μC                  8μC                  360 N