Permutations and CombinationsMCQPYQ June 22Question 1618 of 249
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If four words are taken with or without meaning from the word 'LOGARITHAM' without repetition. How many words will be formed?

Select Your Answer (Keyboard: A, B, C, D)
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Correct Answer

✅ Option b — 2520

All Options:

  • A5040
  • B2520
  • C120
  • D40320

Detailed Solution & Explanation

The word **LOGARITHAM** has 10 letters: L, O, G, A, R, I, T, H, A, M (with 'A' appearing 2 times).

If all 10 letters were distinct (like in the standard word "LOGARITHMS"), the number of 4-letter words would be 10P4=5040\displaystyle ^{10}P_4 = 5040.
Since 'A' is repeated twice, the textbook answer key attempts to adjust for this repetition by dividing the total permutations by $2!:<br><spanclass="katex−display"><spanclass="katex"><spanclass="katex−mathml"><mathxmlns="http://www.w3.org/1998/Math/MathML"display="block"><semantics><mrow><mspacelinebreak="newline"></mspace><mi>t</mi><mi>e</mi><mi>x</mi><mi>t</mi><mrow><mi>W</mi><mi>a</mi><mi>y</mi><mi>s</mi><mostretchy="false">(</mo><mi>a</mi><mi>s</mi><mi>p</mi><mi>e</mi><mi>r</mi><mi>a</mi><mi>n</mi><mi>s</mi><mi>w</mi><mi>e</mi><mi>r</mi><mi>k</mi><mi>e</mi><mi>y</mi><mostretchy="false">)</mo></mrow><mo>=</mo><mspacelinebreak="newline"></mspace><mi>f</mi><mi>r</mi><mi>a</mi><mi>c</mi><mrow><msup><mrow></mrow><mn>10</mn></msup><msub><mi>P</mi><mn>4</mn></msub></mrow><mrow><mn>2</mn><mostretchy="false">!</mo></mrow><mo>=</mo><mspacelinebreak="newline"></mspace><mi>f</mi><mi>r</mi><mi>a</mi><mi>c</mi><mn>50402</mn><mo>=</mo><mn>2520</mn></mrow><annotationencoding="application/x−tex">textWays(asperanswerkey)=frac10P42!=frac50402=2520</annotation></semantics></math></span><spanclass="katex−html"aria−hidden="true"><spanclass="mspacenewline"></span><spanclass="base"><spanclass="strut"style="height:1em;vertical−align:−0.25em;"></span><spanclass="mordmathnormal">t</span><spanclass="mordmathnormal">e</span><spanclass="mordmathnormal">x</span><spanclass="mordmathnormal">t</span><spanclass="mord"><spanclass="mordmathnormal"style="margin−right:0.13889em;">W</span><spanclass="mordmathnormal">a</span><spanclass="mordmathnormal"style="margin−right:0.03588em;">y</span><spanclass="mordmathnormal">s</span><spanclass="mopen">(</span><spanclass="mordmathnormal">a</span><spanclass="mordmathnormal">s</span><spanclass="mordmathnormal">p</span><spanclass="mordmathnormal"style="margin−right:0.02778em;">er</span><spanclass="mordmathnormal">an</span><spanclass="mordmathnormal">s</span><spanclass="mordmathnormal"style="margin−right:0.02691em;">w</span><spanclass="mordmathnormal"style="margin−right:0.02778em;">er</span><spanclass="mordmathnormal"style="margin−right:0.03148em;">k</span><spanclass="mordmathnormal"style="margin−right:0.03588em;">ey</span><spanclass="mclose">)</span></span><spanclass="mspace"style="margin−right:0.2778em;"></span><spanclass="mrel">=</span></span><spanclass="mspacenewline"></span><spanclass="base"><spanclass="strut"style="height:1.0585em;vertical−align:−0.1944em;"></span><spanclass="mordmathnormal"style="margin−right:0.10764em;">f</span><spanclass="mordmathnormal"style="margin−right:0.02778em;">r</span><spanclass="mordmathnormal">a</span><spanclass="mordmathnormal">c</span><spanclass="mord"><spanclass="mord"><span></span><spanclass="msupsub"><spanclass="vlist−t"><spanclass="vlist−r"><spanclass="vlist"style="height:0.8641em;"><spanstyle="top:−3.113em;margin−right:0.05em;"><spanclass="pstrut"style="height:2.7em;"></span><spanclass="sizingreset−size6size3mtight"><spanclass="mordmtight"><spanclass="mordmtight">10</span></span></span></span></span></span></span></span></span><spanclass="mord"><spanclass="mordmathnormal"style="margin−right:0.13889em;">P</span><spanclass="msupsub"><spanclass="vlist−tvlist−t2"><spanclass="vlist−r"><spanclass="vlist"style="height:0.3011em;"><spanstyle="top:−2.55em;margin−left:−0.1389em;margin−right:0.05em;"><spanclass="pstrut"style="height:2.7em;"></span><spanclass="sizingreset−size6size3mtight"><spanclass="mordmtight">4</span></span></span></span><spanclass="vlist−s">​</span></span><spanclass="vlist−r"><spanclass="vlist"style="height:0.15em;"><span></span></span></span></span></span></span></span><spanclass="mord"><spanclass="mord">2</span><spanclass="mclose">!</span></span><spanclass="mspace"style="margin−right:0.2778em;"></span><spanclass="mrel">=</span></span><spanclass="mspacenewline"></span><spanclass="base"><spanclass="strut"style="height:0.8889em;vertical−align:−0.1944em;"></span><spanclass="mordmathnormal"style="margin−right:0.10764em;">f</span><spanclass="mordmathnormal"style="margin−right:0.02778em;">r</span><spanclass="mordmathnormal">a</span><spanclass="mordmathnormal">c</span><spanclass="mord"><spanclass="mord">5040</span></span><spanclass="mord"><spanclass="mord">2</span></span><spanclass="mspace"style="margin−right:0.2778em;"></span><spanclass="mrel">=</span><spanclass="mspace"style="margin−right:0.2778em;"></span></span><spanclass="base"><spanclass="strut"style="height:0.6444em;"></span><spanclass="mord">2520</span></span></span></span></span><br>ThismatchesOptionB.<br><br>Letusalsoshowthemathematicallyrigorousderivationofthisproblem:<br>Thedistinctlettersare\displaystyle :<br><span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mspace linebreak="newline"></mspace><mi>t</mi><mi>e</mi><mi>x</mi><mi>t</mi><mrow><mi>W</mi><mi>a</mi><mi>y</mi><mi>s</mi><mo stretchy="false">(</mo><mi>a</mi><mi>s</mi><mi>p</mi><mi>e</mi><mi>r</mi><mi>a</mi><mi>n</mi><mi>s</mi><mi>w</mi><mi>e</mi><mi>r</mi><mi>k</mi><mi>e</mi><mi>y</mi><mo stretchy="false">)</mo></mrow><mo>=</mo><mspace linebreak="newline"></mspace><mi>f</mi><mi>r</mi><mi>a</mi><mi>c</mi><mrow><msup><mrow></mrow><mn>10</mn></msup><msub><mi>P</mi><mn>4</mn></msub></mrow><mrow><mn>2</mn><mo stretchy="false">!</mo></mrow><mo>=</mo><mspace linebreak="newline"></mspace><mi>f</mi><mi>r</mi><mi>a</mi><mi>c</mi><mn>50402</mn><mo>=</mo><mn>2520</mn></mrow><annotation encoding="application/x-tex">\Ways (as per answer key)\displaystyle \text{Ways (as per answer key)} = \10P42!\displaystyle \frac{^{10}P_4}{2!} = \50402=2520\displaystyle \frac{5040}{2} = 2520</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="mspace newline"></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">t</span><span class="mord mathnormal">e</span><span class="mord mathnormal">x</span><span class="mord mathnormal">t</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.13889em;">W</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mord mathnormal">s</span><span class="mopen">(</span><span class="mord mathnormal">a</span><span class="mord mathnormal">s</span><span class="mord mathnormal">p</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mord mathnormal">an</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:0.02691em;">w</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mord mathnormal" style="margin-right:0.03588em;">ey</span><span class="mclose">)</span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span></span><span class="mspace newline"></span><span class="base"><span class="strut" style="height:1.0585em;vertical-align:-0.1944em;"></span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">a</span><span class="mord mathnormal">c</span><span class="mord"><span class="mord"><span></span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.8641em;"><span style="top:-3.113em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">10</span></span></span></span></span></span></span></span></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.13889em;">P</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.3011em;"><span style="top:-2.55em;margin-left:-0.1389em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight">4</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span></span><span class="mord"><span class="mord">2</span><span class="mclose">!</span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span></span><span class="mspace newline"></span><span class="base"><span class="strut" style="height:0.8889em;vertical-align:-0.1944em;"></span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">a</span><span class="mord mathnormal">c</span><span class="mord"><span class="mord">5040</span></span><span class="mord"><span class="mord">2</span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">2520</span></span></span></span></span><br>This matches Option B.<br><br>Let us also show the mathematically rigorous derivation of this problem:<br>The distinct letters are\{L, O, G, A, R, I, T, H, M\}(9distinct,with′A′havingfrequency2,andothersfrequency1).<br>−∗∗Case1:All4lettersaredistinct.∗∗Weselect4distinctlettersfrom9:\displaystyle (9 distinct, with 'A' having frequency 2, and others frequency 1).<br>- **Case 1: All 4 letters are distinct.** We select 4 distinct letters from 9:^9P_4 = 3024.<br>−∗∗Case2:2lettersare′A′andtheother2aredistinct.∗∗Weselect2otherdistinctlettersfrom8:\displaystyle .<br>- **Case 2: 2 letters are 'A' and the other 2 are distinct.** We select 2 other distinct letters from 8:^8C_2 \×\displaystyle \times \4!2!=28\displaystyle \frac{4!}{2!} = 28 \×\displaystyle \times 12 = 336\displaystyle .<br>- **Total rigorous ways:** &#36;3024 + 336 = 3360.
Since 3360 is not in the options, Option B (2520) is the intended textbook answer based on the simplified division logic.
Hence, **Option B** is the correct answer.

About This Chapter: Permutations and Combinations

Paper

Paper 3: Quantitative Aptitude

Weightage

4-6 Marks

Key Topics

Factorials, Permutations, Combinations

This chapter deals with the fundamental principles of counting. It covers factorials, circular permutations, restricted permutations, combinations, and the differences between selecting items versus arranging them.

View Official ICAI Syllabus

Exam Strategy Tip

The most common mistake is confusing 'P' (Arrangement) with 'C' (Selection). If order matters (like opening a lock), use P. If order doesn't matter (like choosing a team), use C.

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