electricity magnetic physics

Question

4. Two point charges +9e and +1e are kept at a distance of 16 cm from each other. At what point between these charges, should a third charge q to be placed so that it remains in equilibrium?

Answer 1

At equilibrium
Force act on q due to +9e = Force act on q due to +1e
K(+9e)q/r² = k(+1e)q/(16 - r)²
9/r² = 1/(16 - r)²
Taking square root both sides,
3/r = 1/(16 - r)
48 - 3r = r
48 = 4r
r = 12cm