Oh snap, this one was pretty tricky (if for no other reason than the crazy Sphinxlike wording!) but I got D.
Let T=the total number of lights; F=the number of lights that should be oFF, and N the number of lights that should be oN.0.8T=0.4F+0.9N
is the mathematical statement of that riddle. Conservation of light bulbs
Substituting one expression into the other, I foundF=0.2T
Returning to the first equation,0.8T=0.08T+0.9N
So the fraction that should be off is a 10% contribution to the total number on.
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