UPSC Prelims 2013 · CSAT

Question 50: Logical Reasoning and Mental Ability

Examine the following three figures in which the numbers follow a specific pattern :

The missing number (?) in the third figure above is

three figures each made of three boxes in an L shape: the first has 84 on top and 14, 12 below; the second has 81 on top and 18, 9 below; the third has 88 on top and ?, 11 below

b16

c21

d28

AnswerUPSC’s answer is (b).

In each figure the top number is half the product of the two bottom numbers: 14 x 12 / 2 = 84, and 18 x 9 / 2 = 81. Adding or subtracting the bottom numbers does not reproduce the top, so the product rule is the one to use. For the third figure, ? x 11 / 2 = 88, so ? x 11 = 176 and ? = 16.

Why each option is right or wrong

(a) Wrong. 7 x 11 / 2 = 38.5, not 88.
(b) Right. 16 x 11 / 2 = 88.
(c) Wrong. 21 x 11 / 2 = 115.5, not 88.
(d) Wrong. 28 x 11 / 2 = 154, not 88.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2013, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.