Question

Eric's notebook has 30 pages he wants to write a number on each page how many digits will he right when he numbers all the pages in the notebook

Answer 1

 51 Because if there was two numbers for each page there would be 60 page numbers but there is pages 1-9 so you subtract 9 from 60 to get your answer.

Answer 2

pages 1-9 have 1 digit each
pages 10-30 have 2 digits each
so 9 pages have 1 and 21 pages have 2
9×1+21×2=9+42=51