UPSC Prelims 2019 CSAT Paper II · Q70 of 79 Mental Ability easy

In the sequence 1, 5, 7, 3, 5, 7, 4, 3, 5, 7, how many such 5s are there which are not immediately preceded by 3 but are immediately followed by 7 ?

  1. (a) 1 ✓ UPSC's answer
  2. (b) 2
  3. (c) 3
  4. (d) None

Why the answer is (a)

• The sequence is: 1, 5, 7, 3, 5, 7, 4, 3, 5, 7.

• We need to find 5s that are immediately followed by 7.

• The 5s followed by 7 are at positions 2 (1, 5, 7), 5 (3, 5, 7), and 9 (3, 5, 7).

• Check the preceding number for each: Position 2 is preceded by 1 (not 3), so it qualifies.

• Position 5 is preceded by 3, so it does not qualify.

• Position 9 is preceded by 3, so it does not qualify.

• Only one 5 meets both conditions, so the answer is 1.

Why the other options are wrong

(b) 2
There are only two 5s followed by 7 that are not preceded by 3 if one miscounts, but actually only one qualifies.
(c) 3
There are only three 5s followed by 7 in total, but two of them are preceded by 3, leaving only one valid case.
(d) None
There is exactly one 5 that satisfies the condition, so 'None' is incorrect.

Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2019, held on 2 June 2019. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.

Reading the answer is not the same as getting it right under a clock. Practise this question with UPSC's negative marking, and anything you miss goes into an error notebook until you get it right twice.

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