Bunuel
If x is a positive integer and x^3 is a multiple of by 81, which of the following could be the remainder when x is divided by 36?
I. 0
II. 3
III. 27
A. I only
B. II only
C. III only
D. I and III only
E. I, II, and III
This is a interesting question. When we think about xcube being multiple of 81. We can write "x = 9 x (N/9)_1/3". But since in question it is already cleared that x is a positive integer which means " (N/9)_1/3" is a integer aswell. Now we dont need to worry about " (N/9)_1/3" and focus on " 9 x (N/9)_1" and write it as, 9K where K is any integer. When we divide 9K by 36,
it is clear that remainder would be either 0 when K is any multiple of 4,
remainder will be 9 when K is any number starting 1 with a difference of 4 , like 1, 5, 9, 13, ...., etc.
remainder will be 18 when K is any number starting 2 with a difference of 4, like 2, 6, 10, ... etc.
Lastly, it will have remainder of 27 when K is number starting 3 and have difference of 4, like 3, 7, 11, 15, ... etc.
So we can clearly see, remainder is either 0, 9, 18 or 27. So correct answer is (D)