Q. Consider the following statements : 1. The sum of 5 consecutive integers can be 100. 2. The product of three consecutive natural numbers can be equal to their sum.
which of the above statements is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 Correct Answer: (c) Both 1 and 2
Question from UPSC Prelims 2021 CSAT Paper
Explanation :
Sum & Product of Consecutive Integers
Problem Part 1 – Five Consecutive Integers Let’s say the first integer is x. The next four numbers are: x+1, x+2, x+3, and x+4 The sum of these five numbers is 5x + 10 If this sum equals 100, then: 5x + 10 = 100 Solving for x: x = 18 Therefore, the five consecutive integers are: 18, 19, 20, 21, 22
Problem Part 2 – Numbers 1, 2, and 3 Sum: 1 + 2 + 3 = 6 Product: 1 × 2 × 3 = 6
Conclusion: Both statements are correct since: 1. We found five consecutive integers that sum to 100 2. For numbers 1, 2, and 3, their sum equals their product
Q. On a chess board, in how many different ways can 6 consecutive squares be chosen on the diagonals along a straight path ?
(a) 4 (b) 6 (c) 8 (d) 12 Correct Answer: (b) 6
Question from UPSC Prelims 2021 CSAT Paper
Explanation :
Chess Board Consecutive Squares Chosen
Given: – Chess board has 2 diagonals with 8 squares each – Need to select 6 consecutive squares – These 6 squares must be consecutive along a diagonal
Step 1: Analyze one diagonal – Diagonal has 8 squares – Need to select 6 consecutive squares – Think of 6 squares as one block – This block can slide along diagonal
Step 2: Calculate possible positions on first diagonal – 8 squares total, need 6 consecutive – First position: squares 1-6 – Second position: squares 2-7 – Third position: squares 3-8 Total positions on first diagonal = 3
Step 3: Calculate possible positions on second diagonal – Same calculation as first diagonal – 3 possible positions Total positions on second diagonal = 3
Step 4: Calculate total number of ways – First diagonal: 3 ways – Second diagonal: 3 ways Total ways = 3 + 3 = 6
Q. If the price of an article is decreased by 20% and then the new price is increased by 25%, then what is the net change in the price?
(a) 0% (b) 5% increase (c) 5% decrease (d) Cannot be determined due to insufficient data Correct Answer: (a) 0%
Question from UPSC Prelims 2021 CSAT Paper
Explanation :
Price of an article is decreased & increased by 20, 25
Given: – Original price = P – First change: 20% decrease – Second change: 25% increase
Step 1: Calculate price after 20% decrease – Decrease amount = 20% of P = 0.2P – New price = P – 0.2P – New price = 0.8P
Step 2: Calculate final price after 25% increase – Increase amount = 25% of 0.8P = 0.25(0.8P) = 0.2P – Final price = 0.8P + 0.2P – Final price = 1P
Step 3: Calculate net percentage change – Original price = P – Final price = P – Net change = (Final price – Original price)/Original price × 100 – Net change = (P – P)/P × 100 – Net change = 0%
Q. The difference between a 2-digit number and the number obtained by interchanging the positions of the digits is 54.
1. The sum of the two digits of a number can be determined only if the product of the two digits is known. 2. The difference between the two digits of the number can be determined.
Which of the above statements is/are correct? (a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2 Correct Answer: (b) 2 only
Question from UPSC Prelims 2021 CSAT Paper
Explanation :
The difference between 2 digit number and the number obtained is 54
Given: – Original number = 10x + y (where x is tens digit, y is units digit) – New number after interchanging = 10y + x – Difference between these numbers = 54
Step 1: Write equation for the difference (10x + y) – (10y + x) = 54
Step 2: Simplify left side 10x + y – 10y – x = 54 9x – 9y = 54
Step 3: Further simplify 9(x – y) = 54 x – y = 6
Step 4: Analyze what we know – Difference between tens and units digit is 6 – Since it’s a 2-digit number: * x must be from 1 to 9 * y must be from 0 to 9 * x must be greater than y by 6
Step 5: Find possible digit pairs Since x – y = 6: – If y = 0, x = 6 – If y = 1, x = 7 – If y = 2, x = 8 – If y = 3, x = 9
Analysis:
1. Statement 1 (about sum of digits) is incorrect – Multiple pairs of digits satisfy x – y = 6 – Each pair would give different sums – Examples: 6+0=6, 7+1=8, 8+2=10, 9+3=12
2. Statement 2 (about difference of digits) is correct – We proved x – y = 6 – This is a definite value
Answer: Statement 1 is incorrect and Statement 2 is correct.
Q. A pie diagram shows the percentage distribution of proteins, water and on the dry elements in the human body.
Given that proteins correspond to 16% and water corresponds to 70%. If both proteins and the other dry elements correspond to p%, then what is the central angle of the sector representing p on the pie diagram?
A pie chart represents 100% of the data as a full circle with 360 degrees. In this case, proteins correspond to 16% and water corresponds to 70%, so the other dry elements correspond to 100% – 16% – 70% = 14%. Since p represents the sum of proteins and other dry elements, p = 16% + 14% = 30%.
Since p represents 30% of the data, the central angle of the sector representing p on the pie diagram is (30/100) * 360 = 108 degrees.
Q. A biology class at high school predicted that a local population of animals will double in size every 12 years.
The population at the beginning of the year 2021 was estimated to be 50 animals. If P represents the population after n years, then which one of the following equations represents the model of the class for the population? (a) P = 12 + 50n (b) P = 50 + 12n (c) P = 50(2)^12n (d) P = 50 (2)^(n/12) Correct Answer: (d) P = 50 (2)^(n/12)
Question from UPSC Prelims 2021 CSAT Paper
Explanation :
Biology Class at High School Population Growth Model
Given: – Initial population (2021) = 50 animals – Population doubles every 12 years – Need formula for population P after n years
Step 1: Identify Growth Type – Not linear growth (population doesn’t increase by fixed amount) – Population multiplies by 2 at fixed intervals – This indicates exponential growth
Step 2: Basic Exponential Growth Formula P = Initial Value × (growth factor)^(time/interval)
Step 3: Identify Components – Initial Value = 50 – Growth factor = 2 (doubles) – Time = n years – Interval = 12 years
Step 4: Analyze Options a) P = 12 + 50n Incorrect: Shows linear growth
b) P = 50 + 12n Incorrect: Shows linear growth
c) P = 50(2)^12n Incorrect: Time interval ratio is wrong
d) P = 50(2)^(n/12) Correct: Shows proper exponential growth
Step 5: Verify Option D After 0 years: P = 50(2)^0 = 50 (correct initial population) After 12 years: P = 50(2)^1 = 100 (population doubles) After 24 years: P = 50(2)^2 = 200 (doubles again)
Answer: The correct formula is P = 50(2)^(n/12) This formula accurately represents population doubling every 12 years from an initial value of 50 animals.
Q. From January 1, 2021, the price of petrol (in Rupees per litre) on mth day of the year is 80 + 0.1m, where m = 1, 2, 3, …, 100 and thereafter remains constant. On the other hand, the price of diesel (in Rupees per litre) on nth day of 2021 is 69 + 0.15n for any n. On which date in the year 2021 are the prices of these two fuels equal?
(a) 21st May (b) 20th May (c) 19th May (d) 18th May Correct Answer: (b) 20th May
Question from UPSC Prelims 2021 CSAT Paper
Explanation :
Problem: Finding when petrol and diesel prices are equal ?
Given: – 2021 is non leap year – Petrol price formula: 80 + 0.1m (where m = 1 to 100, constant thereafter) – Diesel price formula: 69 + 0.15n (for any value of n)
Step 1: Set up equation for equal prices 80 + 0.1m = 69 + 0.15n
Step 2: Solve for n in terms of m 80 + 0.1m = 69 + 0.15n 0.1m – 0.15n = -11 -0.15n = -11 – 0.1m n = (2m + 220)/3
Step 3: Find n when m = 100 n = (2(100) + 220)/3 n = (200 + 220)/3 n = 420/3 n = 140
Answer: The prices of petrol and diesel are equal on the 140th day of 2021, which is May 20th.