2020 CSAT, all questions
UPSC Prelims 2020 · CSAT
Question 39: Basic Numeracy
The average age of a teacher and three students is 20 years. If all the three students are of same age and the difference between the age of the teacher and each student is 20 years, then what is the age of the teacher?
a25 years
b30 years
c35 years
d45 years
AnswerUPSC’s answer is (c).
Total age of the four people = 4 × 20 = 80. Let each student be s years old; the teacher is s + 20. Then 3s + s + 20 = 80, so 4s = 60 and s = 15. The teacher is 15 + 20 = 35. Check: (35 + 15 × 3)/4 = 80/4 = 20.
Why each option is right or wrong
- (a) 25 would make each student 5, with a total of 40.
- (b) 30 would make each student 10, with a total of 60.
- (c) Right: teacher 35, students 15, total 80.
- (d) 45 would make each student 25, with a total of 120.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.