Each light bulb at a hotel is either incandescent or fluorescent. At a certain moment, thirty percent of the incandescent bulbs are switched on, and eighty percent of the fluorescent bulbs are switched on. If 40 percent of all the bulbs are switched on at this moment, what percent of the bulbs that are switched on are incandescent?	Choose one.
A) 20% B) 40% C) 50% D) 60% E) 80%
Let I be the number of incandescent bulbs.   Let F be the number of fluorescent bulbs.   0.3I + 0.8F = 0.4(I+F)   0.4F = 0.1I   F = 0.25I   This means that for every 4 incandescent bulbs, there is 1 fluorescent bulb.   The percent of bulbs that are switched on which are incandescent is:   0.3I / (0.3I + 0.8F) = 0.3I / (0.3I + 0.8*0.25I) = 0.3I / 0.5I = 3/5 = 60%.   The answer is 60%.
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Each light bulb at a hotel is either incandescent or fluorescent. At a certain moment, thirty percent of the incandescent bulbs are switched on, and eighty percent of the fluorescent bulbs are switched on. If 40 percent of all the bulbs are switched on at this moment, what percent of the bulbs that are switched on are incandescent?