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Review: Contaminated Volume in a Tank
Explanation
The volume of this contaminated tank is 
water and 
air. A third of the water volume, which is of the total, is contaminated, and a fifth of
the air volume is contaminated, so the contaminated fraction is:
If the setup isn't clear, you can clarify it (or avoid it)
by considering the case in which the total volume is 60 (60 is often useful as
an example when you have to do lots of dividing, because it's divisible by so
many small numbers). Then 45 of the volume is water, and 15 of that 45 is
contaminated; similarly, you'll find a volume of 3 for the air is polluted, for
a total of 
,
which is 
and hence gives the same result.
The correct answer is (B).
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