What is the force on a $1\mathrm{.}25C$ charged quarter placed $15cm$ from another quarter with $-9\mathrm{.}86C$ of charge?

How far away would the quarters in #$9$ be if the force on them is $-19\mathrm{.}7N?$

What do you notice about the distance ratio in #$9$ and #$10?$

What do you notice about the force ratio in #$9$ and #$10?$