document.write( "Question 763934: please help me how many five digits numbers can be formed from 1,9,4,2 and 7 if 4 and 2 are always to be together.
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Algebra.Com's Answer #465112 by ramkikk66(644)\"\" \"About 
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document.write( "Your question:\r\n" );
document.write( "How many five digits numbers can be formed from 1,9,4,2 and 7 \r\n" );
document.write( "if 4 and 2 are always to be together.\r\n" );
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document.write( "Solution:\r\n" );
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document.write( "I am assuming that each digit can be used only once.\r\n" );
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document.write( "If 4 and 2 are to be always together, consider them as one unit. \r\n" );
document.write( "The other 3 numbers form 3 units.\r\n" );
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document.write( "Remember the principles of permutations. \r\n" );
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document.write( "n things can be arranged in n positions in n! factorial ways. \r\n" );
document.write( "The first position can be filled in n ways, the second in (n-1) ways \r\n" );
document.write( "and so on. So the total number of ways is n*(n-1)*(n-2)...*1 = n!)\r\n" );
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document.write( "In general, if A can be done in n ways, and B can be done in\r\n" );
document.write( "m ways, together A *and* B can be done in n*m ways)\r\n" );
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document.write( "Applying the above rules, the 4 units can be arranged in 4! \r\n" );
document.write( "(factorial 4) ways.\r\n" );
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document.write( "Now the 2 digits which are in the single unit (2,4) can be themselves \r\n" );
document.write( "arranged in 2! or 2 ways. \r\n" );
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document.write( "So the total number of arrangements = 2! * 4! = 2 * 24 = 48.\r\n" );
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document.write( "The total number of 5 digit numbers (keeping 2 and 4 together) = \"highlight%2848%29\".\r\n" );
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