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27 Logarithms PYQ (Solutions)

Master Logarithms for CAT 2026 with practice questions and detailed explanations

CAT 2025

Logarithms are a recurring algebra topic, often appearing once per exam. They are frequently grouped with exponents/surds.

  • CAT 2017 did not explicitly feature logs beyond maybe a part of an algebra question.
  • CAT 2018–2019 each included a couple of log questions.
    • For instance, CAT 2019’s “Modern Math” had 1 question which could have been a log problem.
  • In CAT 2020, logs showed up significantly – slot 1 had 4 questions involving logs/indices, though other slots had fewer.
  • In CAT 2021–2022, typically 1 log question per slot was observed.

Logs are among the most frequent sub-topics in QA, so expect at least one question on properties of logarithms (or equations using logs) in most CAT papers.

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Weightage Over Past Years

YearQ.NODifficulty Level
20243Hard
20233Hard
20221Medium
20213Hard
20205Hard
20193Medium
20185Hard
20174Medium

CAT 2024 Logarithms questions

Question 1

Slot-1

If xx is a positive real number such that 4log10x+4log100x+8log1000x=134 \log _{10} x+4 \log _{100} x+8 \log _{1000} x=13 , then the greatest integer not exceeding xx , is

Question 2

Slot-2

If a,ba, b and cc are positive real numbers such that a>10bca>10 \geq b \geq c and log8(a+b)log2c+log27(ab)log3c=23\frac{\log _8(a+b)}{\log _2 c}+\frac{\log _{27}(a-b)}{\log _3 c}=\frac{2}{3} , then the greatest possible integer value of aa is

Question 3

Slot-3

If 3a=4,4b=5,5c=6,6d=7,7e=8,8f=9,then the value of the product abcdef is\text{If } 3^a = 4,\quad 4^b = 5,\quad 5^c = 6,\quad 6^d = 7,\quad 7^e = 8,\quad 8^f = 9, \text{then the value of the product } abcdef \text{ is}

CAT 2023 Logarithms questions

Question 1

Slot-1

If xx and yy are positive real numbers such that logx(x2+12)=4\log_{x}\left(x^2+12\right)=4 and 3logyx=13 \log_{y} x=1, then x+yx+y equals

20
11
68
10

Question 2

Slot-2

For some positive real number xx, if log3(x)+logx(25)logx(0.008)=163\log_{\sqrt{3}}(x) + \frac{\log_x(25)}{\log_x(0.008)} = \frac{16}{3}, then the value of log3(3x2)\log_3\left(3 x^2\right) is

Question 3

Slot-3

For a real number xx, if 12,log3(2x9)log34,log5(2x+172)log54\frac{1}{2}, \frac{\log_{3}(2^x - 9)}{\log_{3} 4}, \frac{\log_{5}\left(2^x + \frac{17}{2}\right)}{\log_{5} 4} are in an arithmetic progression, then what is the common difference?

$\log_{4} 7$
$\log_{4}\left(\frac{3}{2}\right)$
$\log_{4}\left(\frac{7}{2}\right)$
$\log_{4}\left(\frac{23}{2}\right)$

CAT 2022 Logarithms questions

Question 1

Slot-2

The number of distinct integer values of nn satisfying 4log2n3log4n<0\frac{4-\log _2 n}{3-\log _4 n}\lt0 , is

CAT 2021 Logarithms questions

Question 1

Slot-1

If 5log101+x+4log101x=log1011x25 - \log_{10} \sqrt{1+x} + 4 \log_{10} \sqrt{1-x} = \log_{10} \frac{1}{\sqrt{1-x^{2}}}, then 100x100x equals

Question 2

Slot-2

If log2 ⁣[3+log3{4+log4(x1)}]2=0 \log_{2}\!\big[\,3 + \log_{3}\{\,4 + \log_{4}(x-1)\,\}\big] - 2 = 0 then 4x4x equals

Question 3

Slot-3

For a real number aa, if log15a+log32a(log15a)(log32a)=4\frac{\log_{15} a + \log_{32} a}{(\log_{15} a)(\log_{32} a)} = 4 then aa must lie in the range

4 < a < 5
3 < a < 4
a > 5
2 < a < 3

CAT 2020 Logarithms questions

Question 1

Slot-1

If log45=(log4y)(log65)\log_{4} 5 = (\log_{4} y) (\log_{6} \sqrt{5}), then yy equals

Question 2

Slot-1

If yy is a negative number such that 2y2log35=5log232^{y^2 \log_3 5} = 5^{\log_2 3}, then yy equals

$$ -\log_{2} 3 $$
$$ \log_{2}\left(\frac{1}{5}\right) = -\log_{2} 5 $$
$$ -\log_{2}\left(\frac{1}{3}\right) = \log_{2} 3 $$
$$ -\log_{2}\left(\frac{1}{5}\right) = \log_{2} 5 $$

Question 3

Slot-2

The value of logaab+logbba\log_{a} \frac{a}{b} + \log_{b} \frac{b}{a}, for 1<ab1 < a \leq b cannot be equal to

-0.5
1
0
-1

Question 4

Slot-3

2×4×8×16(log24)2(log48)3(log816)4\frac{2×4×8×16}{(log_{2}4)^{2}(log_{4}8)^{3}(log_{8}16)^{4}} equals

Question 5

Slot-3

Let loga30=A\log_a 30 = A, loga53=B\log_a \frac{5}{3} = -B and log2a=13\log_2 a = \frac{1}{3}, then log3a\log_3 a equals

$\frac{2}{A+B-3}$
$\frac{A+B-3}{2}$
$\frac{A+B}{2} - 3$
$\frac{2}{A+B} - 3$

CAT 2019 Logarithms questions

Question 1

Slot-1

If (5.55)x=(0.555)y=1000(5.55)^x = (0.555)^y = 1000, then the value of 1x1y\frac{1}{x} - \frac{1}{y} is

1
$\frac{1}{3}$
$\frac{2}{3}$
3

Question 2

Slot-1

Let xx and yy be positive real numbers such that log5(x+y)+log5(xy)=3\log_{5} (x + y) + \log_{5} (x - y) = 3, and log2ylog2x=1log23\log_{2} y - \log_{2} x = 1 - \log_{2} 3. Then xyxy equals

25
150
250
100

Question 3

Slot-2

If x is a real number ,then loge4xx23\sqrt{log_{e}\frac{4x - x^2}{3}} is a real number if and only if

-3 ≤ x ≤ 3
1 ≤ x ≤ 2
1 ≤ x ≤ 3
-1 ≤ x ≤ 3

CAT 2018 Logarithms questions

Question 1

Slot-1

If xx is a positive quantity such that 2x=3log522^x = 3^{\log 5^2}, then xx is equal to:

$\log_{5} 9$
$1 + \log_{5} \frac{3}{5}$
$1 + \log_{3} \left( \frac{5}{3} \right)$
$\log_5 8$

Question 2

Slot-1

If log 2 (5 + log 3 a) = 3 and log 5 (4a + 12 + log 2 b) = 3, then a + b is equal to

32
59
67
40

Question 3

Slot-1

If log1281=p\log_{12} 81 = p, then 3(4p4+p)3 \left( \frac{4 - p}{4 + p} \right) is equal to:

$\log_{2} 8$
$\log_{6} 8$
$\log_{4} 16$
$\log_{6} 16$

Question 4

Slot-2

If p3=q4=r5=s6p^3 = q^4 = r^5 = s^6, then the value of logs(pqr)\log_s (pqr) is equal to

$\frac{24}{5}$
1
$\frac{47}{10}$
$\frac{16}{5}$

Question 5

Slot-2

1log2100\frac{1}{\log_2 100} - 1log4100\frac{1}{\log_4 100} + 1log5100\frac{1}{\log_5 100} - 1log10100\frac{1}{\log_{10} 100} + 1log20100\frac{1}{\log_{20} 100} - 1log25100\frac{1}{\log_{25} 100} + 1log50100\frac{1}{\log_{50} 100} = ?

0
$\frac{1}{2}$
-4
10

CAT 2017 Logarithms questions

Question 1

Slot-1

The value of log0.0085+log3817\log_{0.008} \sqrt{5} + \log_{\sqrt{3}} 81 - 7 is equal to :

$\dfrac{1}{3}$
$\dfrac{2}{3}$
$\dfrac{5}{6}$
$\dfrac{7}{6}$

Question 2

Slot-1

Suppose, log3x=log12y=a\log_{3} x = \log_{12} y = a, where x,yx, y are positive numbers. If GG is the geometric mean of xx and yy, and log6G\log_{6} G is equal to:

√a
2a
$\frac{a}{2}$
a

Question 3

Slot-2

If xx is a real number such that log35=log5(2+x)\log_{3} 5 = \log_{5} (2 + x), then which of the following is true?

0 < x < 3
23 < x < 30
x > 30
3 < x < 23

Question 4

Slot-2

If log(2a×3b×5c)\log(2^a \times 3^b \times 5^c) is the arithmetic mean of log(22×33×5)\log(2^2 \times 3^3 \times 5), log(26×3×57)\log(2^6 \times 3 \times 5^7), and log(2×32×54)\log(2 \times 3^2 \times 5^4), then aa equals (TITA).

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