A kettle is supplied with 12,500 J of energy and transfers 9,000 J usefully to the heating element and water, what is the efficiency of the kettle?

What you are trying to find

Efficiency tells you what fraction of the input energy becomes useful output energy (the rest is wasted, for example to the surroundings).

Choose the correct efficiency equation

Use: $$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}}$$ If you want a percentage, multiply the result by $100\%$.

Substitute the values from the question

Useful energy output $= 9000\ \text{J}$

Total energy input $= 12500\ \text{J}$

$$\text{efficiency} = \frac{9000}{12500} = 0.72$$

Convert to a percentage

$%%DOLLAR%%0.72 \times 100\% = 72\%$$

So, the kettle is $72\%$ efficient.