There is at an error or a typo in the question.
“1KW is 16 pence” should be “1KWh is 16 pence”.
kW is the unit of power not energy.
We pay for the amount of electrical energy we use.
I also wish if we are paying for power instead of energy.
kWh is the unit of electrical energy in this type of question.
Based on the question, the heater is turned on for 5 hours, so how much of electrical energy is used in terms of kWh?
Then the cost of 1 kWh is 16 pence, so the cost of using the heater for 5 hours is?
Hope the guides make sense to you.