The question tells us:
3000>(1/3^n)>30
n is an integer
We need to find the median of all possible values of n.
Its important in such questions to take a moment and plan a course of action before attacking the question.
Let's rewrite the given inequality as:
3000> 3^(-n) >30
Since 3000 is a big number, and finding the power of 3 under 3000 seems like a daunting task, so let's make it a little easier by dividing all terms by three. We get:
1000 > 3^(-n-1) > 10
Now, finding the lower limit is easy, 3^3>10>3^2
Therefore, least possible value of (-n-1) = 3
-n=4
n=-4 (Least possible integer value of 'n')
For the highest possible value, let's do some estimation:
Since 1000 = 100*10, and we need value less than 1000
The closest values lesser than 1000, that are also powers of three are are 81 & 9.
81*9 gives us 729.
For safety, we quickly check that if we were to increase the power by one more, what will happen, and we can see that it will take the product well over 2100 (since 700*3=2100)
Therefore, highest value of 3^(-n-1)=
81*9
=(3^4)*(3^2)
=(3^6)
For highest possible value of 'n':
-n-1=6
-n=7
n=-7.
This gives us four integer values of 'n':
-4,-5,-6,-7
Since we get four possible values, median will be:
[-5+(-6)]/2
=-11/2
Answer: D