Prelims Solution

Q. If 1 litre of water weighs 1 kg, then how many cubic millimeters of water will weigh 0.1 gm?

Q. If 1 litre of water weighs 1 kg, then how many cubic millimeters of water will weigh 0.1 gm?

(a) 1
(b) 10
(c) 100
(d) 1000
Correct Answer: (c) 100

Question from UPSC Prelims 2020 CSAT Paper

Explanation : 

If 1 litre of water weighs 1kg ….

GIVEN CONVERSIONS:
1. Volume Conversion:
– 1 cubic millimeter = 10^-6 liters
– 1 liter = 10^6 cubic millimeters

2. Weight-Volume Relationship:
– 1000 grams (1 kg) = 10^6 cubic millimeters

CALCULATION:
1. To find volume for 0.1 grams:
– If 1000 grams = 10^6 cubic millimeters
– Then 1 gram = 10^3 cubic millimeters
– Therefore, 0.1 grams = 10^2 cubic millimeters
– 0.1 grams = 100 cubic millimeters

Q. If 1 litre of water weighs 1 kg, then how many cubic millimeters of water will weigh 0.1 gm? Read More »

Q. Three Statements S1, S2 and S3 are given below followed by a Question:  S1: C is younger than D, but older than A and B.  S2: D is the oldest.  S3: A is older than B. 

Q. Three Statements S1, S2 and S3 are given below followed by a Question:

S1: C is younger than D, but older than A and B.
S2: D is the oldest.
S3: A is older than B.

Question: Who among A, B, C and D is the youngest?
Which one of the following is correct in respect of the above Statements and the Question?
(a) S1 alone is sufficient to answer the Question.
(b) S1 and S2 together are sufficient to answer the Question.
(c) S2 and S3 together are sufficient to answer the Question.
(d) S1 and S3 together are sufficient to answer the Question.
Correct Answer: (d) S1 and S3 together are sufficient to answer the Question.

Question from UPSC Prelims 2020 CSAT Paper

Explanation : 

C is younger than D

From statement S1 we know that C is younger than D but older than A and B. So the order of their ages is: D > C > A and B.

From statement S3 we know that A is older than B. Combining this information with statement S1, we get the order of their ages as: D > C > A > B.

So from statements S1 and S3 together, we can conclude that B is the youngest among A, B, C and D

Q. Three Statements S1, S2 and S3 are given below followed by a Question:  S1: C is younger than D, but older than A and B.  S2: D is the oldest.  S3: A is older than B.  Read More »

Q. A digit n > 3 is divisible by 3 but not divisible by 6. Which one of the following is divisible by 4?

Q. A digit n > 3 is divisible by 3 but not divisible by 6. Which one of the following is divisible by 4?

(a) 2n
(b) 3n
(c) 2n + 4
(d) 3n+ 1
Correct Answer: (d) 3n+ 1

Question from UPSC Prelims 2020 CSAT Paper

Explanation : 

Digit n > 3 is divisible by 3

Since n is divisible by 3 but not divisible by 6, it means that n must be an odd multiple of 3. Let’s take n=9 as an example.

(a) 2n = 2 * 9 = 18 which is not divisible by 4

(b) 3n = 3 * 9 = 27 which is not divisible by 4

(c) 2n +4 = (2 *9) +4 =22 which is not divisible by 4

(d) 3n +1 = (3 *9)+1=28 which is divisible by 4

So the correct answer is (d) 3n+1.

Q. A digit n > 3 is divisible by 3 but not divisible by 6. Which one of the following is divisible by 4? Read More »

Q. Which one of the following will have minimum change in its value if s is added to both numerator and the denominator of the fractions 2/3, 3/4, 4/5 and 5/6? 

Q. Which one of the following will have minimum change in its value if s is added to both numerator and the denominator of the fractions 2/3, 3/4, 4/5 and 5/6?

(a) 2/3
(b) 3/4
(c) 4/5
(d) 5/6
Correct Answer: (d) 5/6

Question from UPSC Prelims 2020 CSAT Paper

Explanation : 

Let’s solve this step by step:

PROBLEM: Finding which fraction has minimum change when s is added to both numerator and denominator

ORIGINAL FRACTIONS:
2/3, 3/4, 4/5, 5/6

NEW FRACTIONS AFTER ADDING s:
(2 + s)/(3 + s)
(3 + s)/(4 + s)
(4 + s)/(5 + s)
(5 + s)/(6 + s)

CHANGES IN VALUE:
|2/3 – (2 + s)/(3 + s)|
|3/4 – (3 + s)/(4 + s)|
|4/5 – (4 + s)/(5 + s)|
|5/6 – (5 + s)/(6 + s)|

ANALYSIS:
1. When s becomes very large:
– The numerator terms (2,3,4,5) become negligible
– The denominator terms (3,4,5,6) become negligible
– All fractions approach s/s = 1

2. Original values:
2/3 ≈ 0.667
3/4 = 0.750
4/5 = 0.800
5/6 ≈ 0.833

3. Since all fractions approach 1:
– The fraction closest to 1 will have minimum change
– 5/6 is closest to 1 among all given fractions

CONCLUSION: The fraction 5/6 will have the minimum change in value when s is added to both numerator and denominator.

Q. Which one of the following will have minimum change in its value if s is added to both numerator and the denominator of the fractions 2/3, 3/4, 4/5 and 5/6?  Read More »

Q. What is the remainder when 51 x 27 x 35 x 62 x 75 is divided by 100? 

Q. What is the remainder when 51 x 27 x 35 x 62 x 75 is divided by 100?

(a) 50
(b) 25
(c) 5
(d) 1
Correct Answer: (a) 50

Question from UPSC Prelims 2020 CSAT Paper

Explanation :

Remainder when 51*27*35*62*75 divided by 100

To find remainder when divided by 100:
Take ones place digit of each number: 1, 7, 5, 2, 5
Multiply them: 1 × 7 × 5 × 2 × 5 = 350
Find remainder when 350 ÷ 100 = 50 (remainder)

Therefore, when 51 × 27 × 35 × 62 × 75 is divided by 100, the remainder is 50.

Answer: (a) 50

Q. What is the remainder when 51 x 27 x 35 x 62 x 75 is divided by 100?  Read More »

Q. What is the least four-digit number when divided by 3, 4, 5 and 6 leaves a remainder 2 in each case?

Q. What is the least four-digit number when divided by 3, 4, 5 and 6 leaves a remainder 2 in each case?

(a) 1012
(b) 1022
(c) 1122
(d) 1222
Correct Answer: (b) 1022

Question from UPSC Prelims 2020 CSAT Paper

Explanation :

Let’s solve this problem step by step:

Finding the least four-digit number with remainder 2 when divided by 3, 4, 5, and 6

STEP 1: Find the LCM of 3, 4, 5, and 6
– First, we need the least common multiple (LCM) of 3, 4, 5, and 6
– The LCM of these numbers is 60

STEP 2: Find the smallest four-digit number divisible by LCM
– We need to find the smallest four-digit number divisible by 60
– The smallest four-digit number divisible by 60 is 1020

STEP 3: Add remainder
– Since we want remainder 2 when divided by 3, 4, 5, and 6
– We add 2 to our number: 1020 + 2 = 1022

ANSWER: 1022 is the least four-digit number that leaves remainder 2 when divided by 3, 4, 5, and 6

VERIFICATION:
1022 ÷ 3 = 340 remainder 2
1022 ÷ 4 = 255 remainder 2
1022 ÷ 5 = 204 remainder 2
1022 ÷ 6 = 170 remainder 2

Q. What is the least four-digit number when divided by 3, 4, 5 and 6 leaves a remainder 2 in each case? Read More »

Q. The recurring decimal representation 1.272727… is equivalent to 

Q. The recurring decimal representation 1.272727… is equivalent to

(a) 13/11
(b) 14/11
(c) 127/99
(d) 137/99
Correct Answer: (b) 14/11

Question from UPSC Prelims 2020 CSAT Paper

Explanation :

Recurring Decimal 1.272727…

To solve this problem, let x = 1.272727…

Then, 100x = 127.272727…

Subtracting x from 100x, we get:

  • 99x = 126
  • x = 126/99

We can simplify this fraction by dividing the numerator and denominator by their greatest common factor, which is 3:

x = (126/3)/(99/3) = 42/33

We can simplify this fraction further by dividing the numerator and denominator by their greatest common factor, which is 3:

x = (42/3)/(33/3) = 14/11

Therefore, the recurring decimal representation 1.272727… is equivalent to 14/11, and the correct answer is (b).

Q. The recurring decimal representation 1.272727… is equivalent to  Read More »

Q. Consider the following data: For which period was the natural growth rate maximum? 

Q. Consider the following data: For which period was the natural growth rate maximum?

Year : Birth rate – Death rate
1911-1921 : 48.1 – 35.5
1921-1931 : 46.4 – 36.3
1931-1941 : 45.2 – 31.2
1941-1951 : 39.9 – 27.4
1951-1961 : 41.7 – 22.8
1961-1971 : 41.1 – 18.9
1971-1981 : 37.1 – 14.8

(a) 1911 – 1921
(b) 1941-1951
(c) 1961 – 1971
(d) 1971-1981
Correct Answer: (d) 1971-1981

Question from UPSC Prelims 2020 CSAT Paper

Explanation :

For which period was the natural growth rate maximum ?

The natural growth rate is calculated by subtracting the death rate from the birth rate. From the data you provided, we can calculate the natural growth rates for each period:

  • 1911-1921: 48.1 – 35.5 = 12.6
  • 1921-1931: 46.4 – 36.3 = 10.1
  • 1931-1941: 45.2 – 31.2 = 14
  • 1941-1951: 39.9 – 27.4 = 12.5
  • 1951-1961: 41.7 – 22.8 = 18.9
  • 1961-1971: 41.1 – 18.9 = 22.2
  • 1971-1981: 37.1 -14 .8 = 22.3

The maximum natural growth rate was 22.3 during the period 1971-1981.

Q. Consider the following data: For which period was the natural growth rate maximum?  Read More »

Q. What is the largest number among the following? 

Q. What is the largest number among the following? 

(a) (1/2)^-6
(b) (1/4)^-3
(c) (1/3)^-4
(d) (1/6)^-2
Correct Answer: (c) (1/3)^-4

Question from UPSC Prelims 2020 CSAT Paper

Explanation : 

Largest Number

Let’s solve this problem step by step. A negative exponent means that the base is inverted and the exponent becomes positive. For example, (1/2)^-6 can be written as (2/1)^6.

Using this logic, we can rewrite all the options as follows:
(a) (1/2)^-6 = (2/1)^6 = 64
(b) (1/4)^-3 = (4/1)^3 = 64
(c) (1/3)^-4 = (3/1)^4 = 81
(d) (1/6)^-2 = (6/1)^2 = 36

So among these options, the largest number is 81.

So (c) (1/3)^-4 is indeed the correct answer.

Q. What is the largest number among the following?  Read More »

Q. A shop owner offers the following discount options on an article to a customer:

Q. A shop owner offers the following discount options on an article to a customer:

1. Successive discounts or 10% and 20%, and then pay a service tax of 10%
2. Successive discounts of 20% and 10%, and then pay a service tax of 10%
3. Pay a service tax or 10% first, then successive discounts of 20% and 10%

Which one of the following is correct?
(a) 1 only is the best option for the customer.
(b) 2 only is the best option for the customer.
(c) 3 only is the best option for the customer.
(d) All the options are equally good for the customer.
Correct Answer: (d) All the options are equally good for the customer.

Question from UPSC Prelims 2020 CSAT Paper

Explanation :

A shop owner offers the following …

Let’s calculate the effective discount for each option, assuming the original price of the article is P.

Option 1:

First discount: 10% -> P * (1 – 0.10) = 0.9P
Second discount: 20% -> 0.9P * (1 – 0.20) = 0.9P * 0.8 = 0.72P
Service tax: 10% -> 0.72P * (1 + 0.10) = 0.72P * 1.1 = 0.792P

Option 2:

First discount: 20% -> P * (1 – 0.20) = 0.8P
Second discount: 10% -> 0.8P * (1 – 0.10) = 0.8P * 0.9 = 0.72P
Service tax: 10% -> 0.72P * (1 + 0.10) = 0.72P * 1.1 = 0.792P

Option 3:

Service tax: 10% -> P * (1 + 0.10) = 1.1P
First discount: 20% -> 1.1P * (1 – 0.20) = 1.1P * 0.8 = 0.88P
Second discount: 10% -> 0.88P * (1 – 0.10) = 0.88P * 0.9 = 0.792P

All three options result in the customer paying the same final price, which is 79.2% of the original price (0.792P). So, all options are equivalent in terms of the final price for the customer.

Q. A shop owner offers the following discount options on an article to a customer: Read More »