UPSC Prelims 2019 CSAT Paper II · Q40 of 79 Basic Numeracy easy

The average marks of 100 students are given to be 40. It was found later that marks of one student were 53 which were misread as 83. The corrected mean marks are

  1. (a) 39
  2. (b) 39·7 ✓ UPSC's answer
  3. (c) 40
  4. (d) 40·3

Why the answer is (b)

• Original total marks = 100 × 40 = 4000.

• The misread entry added 83 instead of 53, overstating total by 83 − 53 = 30.

• Corrected total = 4000 − 30 = 3970.

• Corrected mean = 3970 ÷ 100 = 39.7.

• Therefore the official key option (b) follows.

Why the other options are wrong

(a) 39
39 would require total marks of 3900, but the corrected total is 3970.
(c) 40
40 would require no change in total marks, but the total decreases by 30.
(d) 40·3
40.3 would require total marks of 4030, but the corrected total is 3970.

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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