(c) For pair of change $U = \frac{1}{4\pi\varepsilon_{0}}.\frac{q_{1}q_{2}}{r}$
$U_{system} = \frac{1}{4\pi\varepsilon_{0}}\ \left\lbrack \frac{10 \times 10^{- 6} \times 10 \times 10^{- 6}}{10/100} \right.\ $
$\left. \ + \frac{10 \times 10^{- 6} \times 10 \times 10^{- 6}}{10/100} + \frac{10 \times 10^{- 6} \times 10 \times 10^{- 6}}{10/100} \right\rbrack$
$= 3 \times 9 \times 10^{9} \times \frac{100 \times 10^{- 12} \times 100}{10} = 27J$