document.write( "Question 1158513: Determine whether the given credit card number is valid number.
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Algebra.Com's Answer #781458 by math_helper(2461)\"\" \"About 
You can put this solution on YOUR website!
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document.write( "I will do #1, as you are supposed to post one problem per post.  You can use this as a template for problem #2.\r
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document.write( "1. 4417-5486-1785-6411\r
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document.write( "NOTE: check digit is last digit (=1)\r
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document.write( "Drop last digit (the check digit):
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document.write( "  4417-5486-1785-641\r
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document.write( "Reverse digits:
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document.write( "  146-5871-6845-7144\r
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document.write( "Multiply odd-positions by 2:
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document.write( "  24(12)-5(16)72-6(16)4(10)-7248\r
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document.write( "Re-write mod-9:
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document.write( "  243-5772-6741-7248\r
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document.write( "Add all digits:
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document.write( "  2+4+3+5+7+7+2+6+7+4+1+7+2+4+8 = 69\r
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document.write( "Find the number that sums with 69 to make a multiple of 10:  1+69=70, 1 matches the check number.  EDIT (added this): Or, perhaps better, take the check digit (1) add to the above sum (69) to get 70, then check 70 mod 10.  If 0 then the credit card number is valid.\r
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document.write( "The number IS VALID.\r
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