Initial charge on 900pF capacitor will be q=CV
When the two equal capacitors are joined together as shown in figure (b). The charge will be equally divided among them. Therefore,
q′=2q=2CV
Now the energy stored in them will be
E=2C(q′)2+2C(q′)2
⇒E=(2CV)2×C1=4CV2=4900×10−12×(100)2
⇒E=2.25×10−6J