Permutations & Combinations
Dictionary Order
Grade 11

Question:

<p>If all the permutations of the letters in the word OBJECT are arranged (and numbered serially) in alphabetical order as in a dictionary, then the 717th word is</p>
<p>(1) TOJECB</p>
<p>(2) TOEJBC</p>
<p>(3) TOCJEB</p>
<p>(4) TOJCBE</p>

Step-by-Step Solution

Key Concept: Use the counting principle to find which letter occupies each position by systematically eliminating permutations. For each position, count how many valid permutations start with each letter in alphabetical order until we reach the 717th permutation.
<p><strong>Step 1:</strong> Letters in OBJECT in alphabetical order: B, C, E, J, O, T (6 letters, all distinct)</p><p><strong>Step 2:</strong> Count permutations starting with each letter (remaining 5! = 120 each):</p><ul><li>Starting with B: positions 1-120</li><li>Starting with C: positions 121-240</li><li>Starting with E: positions 241-360</li><li>Starting with J: positions 361-480</li><li>Starting with O: positions 481-600</li><li>Starting with T: positions 601-720</li></ul><p><strong>Step 3:</strong> 717th word starts with T (since 601 ≤ 717 ≤ 720). Remaining count: 717 - 600 = 117</p><p><strong>Step 4:</strong> Fix T, remaining letters: B, C, E, J, O (4! = 24 each position):</p><ul><li>TB___: positions 1-24</li><li>TC___: positions 25-48</li><li>TE___: positions 49-72</li><li>TJ___: positions 73-96</li><li>TO___: positions 97-120</li></ul><p>117 falls in TO___ range. Position within this: 117 - 96 = 21</p><p><strong>Step 5:</strong> Fix TO, remaining letters: B, C, E, J (3! = 6 each):</p><ul><li>TOB__: positions 1-6</li><li>TOC__: positions 7-12</li><li>TOE__: positions 13-18</li><li>TOJ__: positions 19-24</li></ul><p>21 falls in TOJ__ range. Position within: 21 - 18 = 3</p><p><strong>Step 6:</strong> Fix TOJ, remaining letters: B, C, E (2! = 2 each):</p><ul><li>TOJB_: positions 1-2</li><li>TOJC_: positions 3-4</li></ul><p>3 falls in TOJC_ range. Position within: 3 - 2 = 1</p><p><strong>Step 7:</strong> Fix TOJC, remaining letters: B, E (1! = 1 each):</p><ul><li>TOJCB: position 1</li><li>TOJCE: position 2</li></ul><p>1st position gives TOJCBE</p><p>∴ Answer: <strong>TOJCBE</strong></p>
Correct Answer: D

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