Leslie Green asks:

Peter changes his old fashioned single 100W incandescent light bulb to a super efficient 4 LED bulb system which gives the same light output, but only consumes 20W in total.

Estimate the saving per year if the bulbs are switched on for 8 hours every day of the year and you pay $0.1 per kWh.

(1 kWh is 1000 Watts for 1 hour)