In order to calculate the product of charges, we can use the formula below for the force acting on two charges (Coulomb law):
![|F|=k_e\frac{|q_1q_2|_{}}{r^2}](https://img.qammunity.org/2023/formulas/physics/college/eun70u8lfqgy6slfio78sork2325kqhs3g.png)
Where ke is the Coulomb constant (ke = 9 * 10^9), q1 and q2 are the charges and r is the distance between the charges.
So, for r = 1 and F = 9 * 10^9, we have:
![\begin{gathered} 9\cdot10^9=9\cdot10^9(|q_1q_2|)/(1^2) \\ |q_1q_2|=1 \end{gathered}](https://img.qammunity.org/2023/formulas/physics/college/tmlgiatkk0yrpdfomya9okqighcr2413tr.png)
Therefore the product of the charges is 1 Coulomb.