A textbook has 1,000 pages. How many of the pages have page numbers whose digits add up to exactly 4?
I approached by going by the number of digits.
1 digit: 4
2 digit: 13, 22,31, 40
3 digit: 103,130, 301,310 | 112, 121, 211 | 400
Am I missing anything because 13 is apparently not the correct answer?
