I got 5,789 because there are 9 one digit numbers. 1-9 -> 9x1=9. There are 90 two digit numbers. (10-99)-> 90x2=180. There are 900 three digit numbers. (100-999)-> 900x3=2700. There are 725 four digit numbers. (1000-1724)-> 724x4= 2900. Then, I added 9+180+2700+2900=5789.
In conclusion, the total number of digits required in numbering a book that has 1724 pages is 5789.
