document.write( "Question 1132040: How many four-digit numbers can be formed using the digits 0,1,2,3,4,5,6,7,8, and 9 if the first digit cannot be 0? Repeated digits are allowed. \n" ); document.write( "

Algebra.Com's Answer #748870 by ikleyn(52781) $\"\"$ $\"About$

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document.write( "It can be calculated and presented by different ways.\r\n" );
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document.write( "The simplest and most straightforward way is to notice that this set of numbers is EXACTLY the set of all 4-digit numbers from 1000 to 9999, \r\n" );
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document.write( "which consists of  9999 - 999 = 9000 numbers.     ANSWER\r\n" );
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document.write( "Another way is to count them as follows:\r\n" );
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document.write( "    Any of 9 digits from 1 to 9 in the most left position.     9 options\r\n" );
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document.write( "    Any of 10 digits from 0 to 9 in the next position.        10 options.\r\n" );
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document.write( "    ---------------------- \" ------------------------         10 options\r\n" );
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document.write( "    ---------------------- \" ------------------------         10 options\r\n" );
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document.write( "In all, there are  9*10^3 = 9000 such numbers.    The same answer.\r\n" );
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