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You can put this solution on YOUR website!
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document.write( "Since they are 4 digit combinations rather than 4-digit numbers,\r\n" );
document.write( "they do not have to begin with something other than 0.\r\n" );
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document.write( "Since they are combinations rather than permutations, the order \r\n" );
document.write( "they are listed in does not matter.\r\n" );
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document.write( "You may have meant something else, but this is the answer to\r\n" );
document.write( "the way it is stated.\r\n" );
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document.write( "282303391 or, in order\r\n" );
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document.write( "012233389.\r\n" );
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document.write( "one 0, one 1, two 2's, three 3's, one 8, one 9.\r\n" );
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document.write( "Case 1: All the digits are different.\r\n" );
document.write( "Choose 4 from these 6 digits: 0,1,2,3,8,9\r\n" );
document.write( "6C4 = 15\r\n" );
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document.write( "[FYI, Those 15 combinations of 4 digits, not permutations\r\n" );
document.write( "and not 4-digit numbers, are \r\n" );
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document.write( "{0,1,2,3}, {0,1,2,8}, {0,1,2,9}, {0,1,3,8}, {0,1,3,9},\r\n" );
document.write( "{0,1,8,9}, {0,2,3,8}, {0,2,3,9}, {0,2,8,9}, {0,3,8,9},\r\n" );
document.write( "{1,2,3,8}, {1,2,3,9}, {1,2,8,9}, {1,3,8,9}, {2,3,8,9}]\r\n" );
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document.write( "Case 2: There are exactly 2 identical digits.\r\n" );
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document.write( "Choose the digit that we have 2 of in 2 ways, as 2 or 3.\r\n" );
document.write( "Choose two other digits in 5C2 ways. \r\n" );
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document.write( "That's 2*5C2 = 20 ways\r\n" );
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document.write( "Case 3: There is only one combination that has 2 pairs of\r\n" );
document.write( "identical digits, {2,2,3,3}\r\n" );
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document.write( "That's 1.\r\n" );
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document.write( "Case 4: 3 identical digits.\r\n" );
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document.write( "Choose the identical digit only 1 way, as 3.\r\n" );
document.write( "Choose the 4th digit 5 ways 0,1,2,8, or 9\r\n" );
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document.write( "That's 1*5 = 5 ways,\r\n" );
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document.write( "[FYI, they are {0,3,3,3}, {1,3,3,3}, {2,3,3,3}, {3,3,3,8}, {3,3,3,9}\r\n" );
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document.write( "Grand total: 15+20+1+5 = 41\r\n" );
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document.write( "Edwin \n" );
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