An engine has an energy input of 125 J, and 35 J of that energy is transformed into useful energy. What is the efficiency of the engine?

Efficiency is equal to 35 / 125 * 100 which is equal to 28%.

Efficiency is the ratio of the useful work performed by a machine or in a process to the total energy that it consumed.

So basically it is the useful output which in this case was 35 J divided by the total input given to the machine which was 125 J in this case multiplying by 100 to get the percentage.

If we did not need the percentage then we could have simply not multiplied it by 100.

Efficiency = (useful output) / (input)

Efficiency = (35 J) / (125 J) = 0.28 = 28%