Answer
$K_f=4.5J$
Work Step by Step
We know that:
$dU=-dK$
$\frac{KqQ}{r_f}-\frac{KqQ}{r_i}=-(K_f-K_i)$
We plug in the known values to obtain:
$\frac{9\times 10^9\times 7.5\times 10^{-6}\times 20\times 10^{-6}}{0.20}-\frac{9\times 10^9\times 7.5\times 10^{-6}\times 20\times 10^{-6}}{0.60}=-(K_f-0)$
$K_f=4.5J$
