Between 1970 and 2007,* commercial books were published with an ISBN-10 (International Standard Book Number) consisting of 10 digits in the format a-bcd-efghi-j. The last digit j is a check digit, meaning that is takes one of the values 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, X (where X is a single letter symbol for 10) and 10a + 9b + 8c + 7d + 6e + 5f + 4g + 3h + 2i + j = 0 (mod 11). In this way, the check digit makes sure the ISBN number is valid.
Which of these are valid ISBN-10 numbers?
(a) 0-495-56203-5
(b) 4-851-60772-4
(c) 2-015-39351-X
(d) 0-618-51471-3
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