Consider the electric force between a pair of charged particles a certain distance apart. By Coulomb's Law, if the charge on one of the particles is doubles, the force is

Doubled as well. Using what I call "Method of Simplest Integers," insert all charges and distance as just 1 (ignore k, because it is a constant), so

Fe=k (1*1) / (1^2) = k (1) / 1=k (1)

Since one of the charges is doubled,

Fe'=k (2*1) / (1^2) 'k (2) / 1=k (2)

Set Fe and Fe' side by side and compare their outcomes. cancel out k, because it's constant, and find out what multiple makes Fe becomes Fe'. In this case, it is 2. So the force would be doubled if one of the charges is doubled.