Two point charges experience a force of repulsion equal to 3

Two point charges experience a force of repulsion equal to 3 times 10^-4 N when separated by 0.4m. If one of the charges is 5 x l0^-4 C, what is the magnitude of the other charge?

Solution

Force between two point charges = Kq1q2/(r*r)

K=8.9 *10^9

so force = 8.9 * 10 ^9 * 5 * 10 ^-4 * q2/.4*.4

so q2= 3*10^-4*4*4/(8.9*10^9*5*10^-4) = 1.07 * 10^-9 C