Two charges $10\mu C$ and $12 \mu C$ are 10cm apart in vacuum.the work done in bringing them 4cm closer will be 

7.2 J

$\text{work done in bringing them closer equal to difference in potential energies}$

$ W = U_f - U_i = \frac{1}{4\pi \epsilon_0} \frac{q_1 q_2}{r_f} - \frac{1}{4\pi \epsilon_0} \frac{q_1 q_2}{r_i} = \frac{1}{4\pi \epsilon_0} q_1q_2(\frac{1}{r_f}-\frac{1}{r_i}) = 7.2J$

$r_i = 10cm , r_f = 6cm$