The force between two charges is given by Coulomb's law:
$$F = k\frac{q_1q_2}{r^2}$$
where k is a constant, q1 and q2 are the charges, and r is the distance between the charges.
So the force acting on 2 charges 1 and 5 will be:
$$F1 = k\frac{1\cdot1}{r^2}$$
The force acting on 2 charges 1 and 25 will be:
$$F2 = k\frac{1\cdot25}{r^2}$$
Therefore the ratio of the force will be:
$$\frac{F1}{F2}=\frac{1/r^2}{25/r^2}=\frac{1}{25}$$
In order for the ratio to be 11, we can have 11 charges on one side and 25 on the other side.
Then the ratio will be:
$$\frac{F11}{F25}=\frac{11/r^2}{25/r^2}=\frac{11}{25}$$
