(5 bulbs)·(60 watts/bulb)·(20 minutes)·(1 joule per sec/watt)·(60 sec/minute)
= (5·60·20·1·60) (bulb·watt·minute·joule per sec·sec) / (bulb·watt·minute)
= 360,000 joules
_____________________________________________
Another way:
Five 60-watt bulbs = 300 watts = 0.3 kilowatt
20 minutes = 1/3 hour
(0.3 kilowatt) · (1/3 hour) = 0.1 kilowatt-hour
