The speed of an aircraft is sometimes expressed as a mach number: mach 1 means that the speed is equal to the speed of sound. if you wish to determine the speed of an aircraft going mach 2.2, for example, in meters per second or miles per hour, what additional information do you need?

The speed of sound needs to be given, in the proper form. This will allow for the proper conversion (namely, a multiplication by the Mach rate) to find the actual speed that the aircraft is traveling, compared to how fast sound travels.

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