N^2 - 8 is the dividend
33 is the divisor
let's take Q as the quotient and R as the remainder.
so i can create a equation: n^2 - 8 = 33Q + R
Which also equals to n^2 = 33Q + R + 8
That means the value of 33Q + R + 8 is a perfect square.
Now i thought let's see the factors of 33 - that's 3 & 11.
If we look at perfect squares:
1^2 = 1 when divided by 3 leaves a remainder 1
2^2 = 4 when divided by 3 leaves a remainder 1
3^2 = 9 when divided by 3 leaves a remainder 0
4^2 = 16 when divided by 3 leaves a remainder 1
5^2 = 25 when divided by 3 leaves a remainder 1
....and so forth.
So basically, any perfect square when divided by 3 should leave a remainder 1 or 0
In that case our RHS should also give us a remainder 1 or 0 when divided by 3, for it to satisfy the equation of N^2
So, trying all the cases given in the question:
Case I. R = 0
So, the equation becomes - n^2 = 33Q + 8 + 0
I know 33Q is divisible by 3, but 8 when divided by 3 leaves a remainder 2 which means, in this equation, it won't be a perfect square.
Case II. R = 15
the equation becomes - n^2 = 33Q + 8 + 15 = 33Q + 23
23 when divided by 3 leaves a remainder 2. So again not a perfect square. This remainder is also not possible.
Case III. R = 21
the equation becomes - n^2 = 33Q + 8 + 21 = 33Q + 29
29 when divided by 3 leaves a remainder 2. So not a perfect square either. This remainder is not possible either.
Thus, the answer is E. None of these.
Just for better understanding of what we did to solve this, we can also take another example. So for instance n = 9 and n^2 = 81
We can break 81 = 33 x 2 + 8 + 7, this is the same as saying n^2 = 33Q + 8 + R, with Q = 2 and R = 7
In this example, 33 x 2, when divided by 3, leaves the remainder 0
and 8+7 = 15, when divided by 3, also leaves a remainder 0
So we can conclude that our RHS is a perfect square.
Bunuel
If n is a positive integer, which of the following could be the remainder when n^2 - 8 is divided by 33?
I. 0
II. 15
III. 21
A. I only
B. II only
C. III only
D. II and III only
E. None of these
⚠️ Important: GMAT Club does not allow AI-generated posts. AI-generated solutions are not eligible for kudos, and users who post them may face moderation action, including a ban.