Suppose we have a large number of identical particles, very small in size. Any of them at 10 cm separation repel with a force of $3 \times {10}^{-10}$ N. If one of them is at 10 cm from a group of n others, how strongly do you expect it to be repelled?

The group of n small charged particles must behave as a single point charge, so that it can have a separation of 10 cm from the one particle in question. Obviously, force on this particle due to the group of n particles is n times the force due to a single particle. Hence, force due to group of n particles, $\mathrm{F} = \mathrm{n} \mathrm{x} 3 \mathrm{x} {10}^{-10} \mathrm{N}$.