UPSC Prelims 2018 · CSAT
Question 38: Basic Numeracy
A bookseller sold 'a' number of Geography textbooks at the rate of ₹ x per book, 'a+2' number of History textbooks at the rate of ₹(x+2) per book and 'a-2' number of Mathematics textbooks at the rate of ₹(x-2) per book. What is his total sale in ₹?
AnswerUPSC’s answer is (b).
Total sale = a·x + (a + 2)(x + 2) + (a - 2)(x - 2). Expand: ax + (ax + 2a + 2x + 4) + (ax - 2a - 2x + 4). The 2a and 2x terms cancel between the second and third products, and the constants add to 8. Total = 3ax + 8. Check with a = 3, x = 5: 15 + 35 + 3 = 53, and 3 × 15 + 8 = 53.
Why each option is right or wrong
- (a) 3x + 3a leaves out the products of count and rate.
- (b) Correct: three products add up to 3ax + 8.
- (c) 9ax is not the sum of the three products.
- (d) x³a³ is a product of the variables, not a sum of sales.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2018, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.