A laptop can convert 400J of electrical energy into a 240J of light and sound. What is the efficiency? Where does the rest of the energy go?

Efficiency = (Wanted) energy out : energy in * 100

Energy in = 400J

Wanted Energy out = 240J

Energy cannot be used up, only transferred, so the remaining energy is most likely to be transferred into unwanted energy (loss of energy) such as heat energy.

Efficiency = 240 : 400 * 100

Efficiency = 0.6 * 100

Efficiency = 60%