I'm assuming that you meant to write 'two 100-w per hour light bulbs used a total of 400w. how many hours did the bulbs run'
so each used 100w per hour and there are 2 light bulbs so in 1 hour 200w is used
200wh times x hour=200wx=400w
divide both sides by 200w
x=2
they ran for 2 hours
