Two similar very small conducting spheres having charge of 40C and -20C are at some distance apart. Now they are . . .

Question : Two similar very small conducting spheres having charge of 40C and -20C are at some distance apart. Now they are touched and then kept at same initial distance. The ratio of the initial to the final force between the two spheres will be ___________. [8:1]

Doubt by Diya

Solution : 

Case I

F = k |40||-20|/r²

F= 800k/r² — (1)

Case II

F' = k |10||10|/r²
F' = 100k/r² — (2)

Dividing (1) by (2)

F/F' = 8/1
F:F' = 8:1

Hence, the required ratio is 8:1