UPSC CSAT Simplified | CSAT a sure shot? Test your aptitude through this Practice Set 5 for Prelims 2025

UPSC CSAT Practice Set 5

—Mudit Gupta

QUESTION #1

Recent studies have revealed that certain species of aphids possess a remarkable ability to manipulate the physiology of their host plants. These aphids secrete a unique blend of chemicals in their saliva that can suppress the plant’s natural defence mechanisms, such as the production of toxins and the release of volatile compounds that attract predatory insects. This allows the aphids to feed more efficiently and reproduce at a faster rate. Researchers believe that understanding the specific compounds involved in this process could lead to breakthroughs in agricultural science, particularly in the development of new strategies for pest control and crop protection.

Which one of the following statements best reflects the most logical, rational, and practical message conveyed by the passage?

A. Farmers can introduce aphids into their fields to naturally control weed growth.

B. This discovery could pave the way for the creation of advanced, environmentally friendly pest control methods.

C. The chemicals in aphid saliva can be used to develop new medications for human diseases.

D. Aphids can be genetically engineered to act as natural predators for other harmful insects.

Explanation:

The passage talks about the special quality of aphids that can be used to suppress the defence mechanism of plants. The passage talks about the scenario wherein this special quality of aphids can be used for R&D in agriculture, thus making agriculture more pest-resistant and thus protecting the crops.

Option A is wrong because nowhere the role of farmers has been mentioned in the passage. Rather, it is the role of R&D to introduce pest-resistant varieties. No duty of farmers has been mentioned.

Option B is correct as this option perfectly captures the essence of the passage regarding the evolving pest-control methods.

Option C is incorrect because the paragraph isn’t talking about human diseases.

Option D is also incorrect as the paragraph isn’t talking about the genetic engineering of aphids but about the pest-control methods that can be developed to protect the crops. 

Therefore, B is the correct answer.

QUESTION #2

Consider the sequence ABC_ _ABC_DABBCD_ABCD that follows a certain pattern. Which of the following completes the sequence? 

A. DACB

B. CDAB

C. DCCA

D. DDCA

Explanation:

Let us write the sequence 4 times and fill all the options respectively:

1. ABCDAABCCDABBCDBABCD – Making the groups of 3/4/5 alphabets, we could not arrive at a logical pattern.

2. ABCCDABCADABBCDBABCD – Similarly, we made the groups of 3/4/5 alphabets here, we could not arrive at a logical pattern.

3. ABCDCABCCDABBCDAABCD  – If we make the group of alphabets here also, no concrete pattern can be formed.

4. ABCDDABCCDABBCDAABCD  – Here, if we make the group of 5 alphabets, we see that a pattern can be formed: ABCDD, ABCCD, ABBCD, AABCD. 

Therefore, D is the correct answer.

QUESTION #3

125 identical cubes are arranged in the form of cubical blocks. How many cubes are surrounded by other cubes from each side?

A. 27

B. 25

C. 21

D. 18

Explanation:

Here is a PYQ from 2024. If there are 125 small cubes, then first of all we have to find out the number of small cubes in one row (n).

Since the value of n is 5, we can say that there are 5 small cubes in one row.

Analysing the question, we have to find out the number of small cubes that are surrounded by other cubes on all the sides, in other words, we have to find the number of cubes that have no faces painted. In other words, we have to find out the number of those cubes that are neither at the edges, nor at the corners, nor at the face, but deep inside the cube, surrounded by other cubes on all the sides.

Number of such cubes that have 0 face painted or that are surrounded by other cubes on all the sides is given by:

Therefore, A  is the correct answer.

QUESTION #4

Let X be a two-digit number and Y be another two-digit number formed by interchanging the digits of X. If (X + Y) is the greatest two-digit number, then what is the number of possible values of X?

A. 2

B. 4

C. 6

D. 8

Explanation:

X is a 2-digit number. Let X = ab

Y is a 2-digit number formed by reversing the digits of X. So, Y = ba

We are given that X+Y is the greatest 2-digit number i.e. X+Y is 99.

Let us write it in a different way: 

X + Y = 99

ab + ba = 99

(We had assumed X = ab and Y = ba) (10a + b) + (a +b)

So, we need to find the values of a and b, such that their sum is 9. The possible values of a and b are: (8,1), (7,2), (6,3), (5,4), (4,5), (3,6), (2,7), (1,8).

We know that X=ab, so all possible values of X are: 81, 72, 63, 54, 45, 36, 27, and 18. So X has 8 possible values.

Therefore, D is the correct answer.

QUESTION #5

In the expression 5*4*3*2*1, *is chosen from +, -, x each at most two times. What is the smallest non-negative value of the expression?

A. 3

B. 2

C. 1

D. 0

Explanation:

Here is a PYQ from 2024. In the above question, certain points need to be kept in mind before solving it:

1. We can choose each symbol out of +, -, and x at most two times and not more than that.

2. We have to use the symbols in such a way that we get the minimum possible non-negative value of the expression. Here the expression non-negative means that the value of the expression can be 0 or positive but it should not have a negative sign.

3. Whenever we have to find out the minimum value, always try and use the subtraction sign with the bigger value. It has a dampening effect on the overall value of the expression.

Solving for the minimum value, we get the equation as: 5 – 4 – 3 + 2 * 1 = 0.

Therefore, D  is the correct answer.

(About our Expert: Mudit Gupta has been a mentor and faculty for the UPSC Civil Services exam for 8 years with expertise in CSAT, Polity, International Relations, and Current Affairs. He is known to break complex concepts into simpler ones that allow UPSC aspirants to develop a grip over the subject matter.)

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