A textbook has 1,000 pages. How many of the pages have page numbers whose digits add up to exactly 4?
+9466 This probably isn't the "smartest" method, but I just wrote them all out here.
I noticed that they follow a kind of pattern. 
004
013 103
022 112 202
031 121 211 301
040 130 220 310 400
I count fifteen of them.
+118609 That is a good method Hectictar, I just want to take a look too :)
400 any order 3 The 4 can be 1st , 2nd or 3rd.
310 any order 3*2=6 ways these pages are 13,31,103,130,301,310
211 any order 3
220 any order 3 ways
15 ways WE agree 
+2441 Well, you can immediately eliminate the possibility of any of the digits being 5,6,7,8, or 9 because that would result in a sum greater than 4, which simplifies the problem somewhat.
