2015 CSAT, all questions
UPSC Prelims 2015 · CSAT
Question 12: Logical Reasoning and Mental Ability
Consider the following matrix :
| 3 | 8 | 10 | 2 | ? | 1 |
| 6 | 56 | 90 | 2 | 20 | 0 |
What is the missing number at ‘?’ in the matrix ?
a5
b0
c7
d3
AnswerUPSC’s answer is (a).
Look at each column: the bottom number equals the top number times one less than itself, n × (n − 1). Check: 3 × 2 = 6, 8 × 7 = 56, 10 × 9 = 90, 2 × 1 = 2, and 1 × 0 = 0. For the column with 20 below the question mark we need n × (n − 1) = 20, which gives n = 5, since 5 × 4 = 20.
Why each option is right or wrong
- (a) Right: 5 × 4 = 20.
- (b) Wrong: 0 × (−1) = 0, not 20.
- (c) Wrong: 7 × 6 = 42, not 20.
- (d) Wrong: 3 × 2 = 6, not 20.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2015, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.