c
\({U_A}\,\, = \,\,{q_A}\,{V_A}\,\, = \,\,10\,\,\left[ {\frac{{k{q_B}}}{{{r_1}}}\,\, + \;\,\frac{{k{q_C}}}{{{r_2}}}} \right]\,\,\, = \,\,10\,\,\left[ {\frac{{10\,\, \times \,40}}{2}\,\, + \;\,\frac{{1\,\, \times \,\,\left( { - 20} \right)}}{4}} \right]\,\, = \,\,150\,\,ergs\)
