First of all - apartments without windows?! I'm pretty sure that those can't legally be called apartments!

But I digress... This problem can also be solved using a DOUBLE-SET MATRIX:



The question is asking for what percentage of apartments with windows have hardwood floors. In other words, (window + hardwood floors) as a proportion of (all apartments with windows):



Now, fill in the information given and make inferences: 50 % apartments in a certain building have windows and hardwood floors.


Next: 25% apartments without windows have hardwood floors.


Make inferences from the final piece of information: 40 % of the apartments do not have hardwood floors
If 40% do not have hardwood floors, 60% do. If 50% have both windows + hardwood, then 10% of apartments have hardwood but no windows. If that's 25% of all apartments without windows, then there must be 40% of apartments without windows.


So, we're looking for 50% of all apartments as a percentage of 60%. We don't even have to do the math - 50 is more than half of 60, so we can eliminate any answer that's 50% or below, leaving us with E. (Or we could do the math: 5/6 = 0.8333...)

The answer is E.