\(\text{You start with a configuration of 3 charges separated by a distance }s\\ \text{By symmetry each pair of charges generates the same energy and thus }\\ \text{each configuration generates }\dfrac{15}{3} = 5J\\ \text{Now we move the charge to the midpoint between two other charges}\\ \text{Our new configuration is one pair separated by distance }s\\ \text{and 2 other pairs separated by distance }\dfrac s 2\)
\(\text{The pair separated by }s \text{ generates }5J \text{ as before}\\ \text{The energy stored is inversely proportional to the distance between them}\\ \text{so halving the distance doubles the energy}\\ \text{thus we have two pairs that generate }2 \cdot 5J = 10J \text{ each}\\ \text{Totaling these up we have } E = (5+10+10) = 25J\)
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+6252 
