Two equal charges Q are placed a certain distance apart. They exert equal-and-opposite forces F on one another. Now one of the charges is doubled in magnitude to 2Q. What happens to the magnitude of the force each charge experiences?

1. Both charges experience forces of magnitude 2F.
2. The Q charge experiences a force of 2F; the 2Q charge experiences a force F.
3. The Q charge experiences a force of F; the 2Q charge experiences a force 2F.
4. None of the above.

Both forces are now 2F. It doesn't matter which charge you consider, the force is given by:

Coulomb's law: F = kQ(2Q) / r²

Another good way to explain this is to remember Newton's Third Law, which says that the forces objects exert on one another are equal-and-opposite.