BTGModeratorVI wrote: ↑Wed Feb 26, 2020 4:23 pm
If S is the sum of the first n positive integers, what is the value of n ?
(1) S < 20
(2) S^2 > 220
Answer:
C
Source: Official Guide
Given: S is the sum of the first n positive integers
Target question: What is the value of n ?
Statement 1: S < 20
If n = 5, then S = 1 + 2 + 3 + 4 + 5 = 15
If n = 6, then S = 1 + 2 + 3 + 4 + 5 + 6 = 21
Since S < 20, we know that n < 6
This means
n COULD equal 1, 2, 3, 4, or 5
Since we cannot answer the
target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: S² > 220
Let's test some values.
If n = 4, then S = 1 + 2 + 3 + 4 = 10, which means S² = 10² = 100
If n = 5, then S = 1 + 2 + 3 + 4 + 5 = 15, which means S² = 15² = 225
Since S² > 220, we know that n > 4
This means
n COULD equal 5 or 6 or 7 or 8 or .....
Since we cannot answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Statements 1 and 2 combined
Statement 1 tells us that n < 6
Statement 2 tells us that n > 4
Combine the inequalities to get: 4 < n < 6
So, it MUST be the case that
n = 5
Since we can answer the
target question with certainty, the combined statements are SUFFICIENT
Answer: C
Cheers,
Brent
