Efficiency is a measure of how much of the energy input is converted into useful output. In this case, the kettle is stated to be 60% efficient, meaning that 60% of the energy input is converted into useful energy transfer to the water.
To find the useful energy transfer to the water, we can use the formula:
Useful energy transfer = Efficiency x Energy input
In this case, we have:
Useful energy transfer = 0.6 x 400 J = 240 J
So, the useful energy transfer to the water would be 240 J.
It means that, the kettle is able to transfer 240 joules of energy to the water, and the remaining 160 J of energy is wasted as heat or other forms of energy.