Call the capacitance C.
Note the energy in a capacitor with voltage V is E =½CV².
Initial energy = ½C(12)² = 72C
40% of energy is delivered, so 60% remains.in the capacitor.
Remaining energy = (60/100) x 72C =43.2C
If the final potential difference is X, the energy stored is ½CX²
½CX² = 43.2C
X² = 2 x 43.2 = 86.4
X = 9.3V
