The librarian at the public library counted the number of books on each shelf. How many shelves have at least 10 books but fewer than 20 books?
Stem | Leaf 
1 | 3, 4
2 | 1, 4, 4, 5
3 | 2, 7
4 | 
5 | 8
Count all the leaves in the row with stem 1.

You counted 2 leaves, which are blue in the stem-and-leaf plot above. 2 shelves have at least 10 books but fewer than 20 books.