How to change the distance between two point charges so that the force of interaction between them increases 16 times.

The two Coulomb forces differ in value by a factor of 16 only due to the change in radius from R to r. Let’s write down the ratio of these forces:
(k * Q1 * Q2) / (r ^ 2) / (k * Q1 * Q2) / (R ^ 2) = 16.
After canceling the same factors:
R ^ 2 / r ^ 2 = 16.
We extract the square root from both parts:
R / r = 4;
r = R / 4.

Answer: To increase the force of interaction of charges by 16 times, you need to reduce the distance between them by 4 times.