Question

a book contains 327 pages. how many times the digits 7 appears in numbering the pages?

Answer 1

07,17,27,37,47,57,67,70,71,72,73,74,75,76,77,77,78,79,87,97,107,117,127,137,147,157,167,170,171,172,173,174,175,176,177,177,178,179,187,197,207,217,227,237,247,257,267,270,271,272,273,274,275,276,277,277,278,279,287,297,307,317,327

there are 63 time 7's printed at the page no. column.

it can also be done by permutation and combination.