UPSC Prelims 2023 · CSAT
Question 20: Basic Numeracy
A, B, C working independently can do a piece of work in 8, 16 and 12 days respectively. A alone works on Monday, B alone works on Tuesday, C alone works on Wednesday; A alone, again works on Thursday and so on. Consider the following statements :
- 1.The work will be finished on Thursday.
- 2.The work will be finished in 10 days.
Which of the above statements is/are correct ?
AnswerUPSC’s answer is (a).
Take 48 units of work: A does 6 a day, B does 3, C does 4, so each 3-day cycle does 13. Nine days (three cycles) finish 39, leaving 9. Day 10 is A's turn: 6 more makes 45. Day 11 is B's turn: 3 more makes 48. The work ends on day 11. Day 1 is Monday, so day 11 is Thursday. Statement 1 is true; it takes 11 days, not 10, so statement 2 is false.
- 1. Day 11 is B's turn and completes the 48 units; day 11 falls on a Thursday.
- 2. After 10 days only 45 of 48 units are done.
How you could have got this
- If you were sure statement 1 is correct, (b) and (d) go: each of them leaves 1 out.
What you needed to know: Day 11 is B's turn and completes the 48 units; day 11 falls on a Thursday.
- Then, if you were sure statement 2 is not correct, (c) goes: it includes 2.
What you needed to know: After 10 days only 45 of 48 units are done.
Only (a) is left. This one needs every statement.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Right: the work finishes on day 11, a Thursday.
- (b) Wrong: the work takes 11 days, not 10.
- (c) Wrong: statement 2 is false.
- (d) Wrong: statement 1 is true.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2023, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.