susana's algebraic solution is spot on. But most people (including me, incidentally) will be able to solve this question more quickly by picking numbers.
When you have a word problem involving fractions, you should start out by picking the lowest common denominator.
So, because the denominators in the question are 4 and 5, we should start with 20.
Let December's revenues be 20.
November's revenue is simply (2/5)*20 = 8.
So, January's revenue is (1/4)*8 = 2.
What is the average of 8 and 2?...(8+2)/2 = 5.
20 is how many times 5?...20/5 = 4.
Thus, the correct answer is 4.
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