homework help!!

[Katie22]

To do with energy and is based around a kettle rated at 2.5kW, taking 2.4 mins to boil.

We know the amount of energy transferred by the kettle every second is 2.5kJ and the amount of energy transferred by the kettle in 2.4 mins is 360kJ but its the next bit we're stuck on:

The amount of useful energy used for heating the water to boiling point

You have to chose from a set of answers so we know the answer is 252 but dont know how they've arrived at it! Any help would be grateful. Its probably really easy but I think we've thought about it too long!
Thanks

[rfhenryuk]

I don;t know whether this will help, but the efficiency of the kettke comes into it. This is the ratio of useful energy to total energy input. Unfortunately, kettles are reckoned to be over 90% efficient, which would give 324 as the answer. 252 corresponds to an efficieny of 70%