An electrical kettle uses 18A of current to boil 500ml of water. What is the electrical resistance of the kettle in ohms?

We can't tell from the given information.Regarding the water, we don't know . . .-- starting temperature-- quantity of water-- time in which the heater boiled it so we don't know how much heat energy it takes to boil it.Even if we DID know the total heat energy required, we don't knowthe voltage supplied to the kettle's heating coil.  Together with thecurrent flowing in the heater, we need to know either its resistanceor the voltage across it in order to calculate the rate at which heat energy is dissipated from the heater.