Final answer:
Alley's fraction representing red and blue tiles should have a numerator of 9 since she has 5 red tiles and 4 blue tiles out of a total of 20 tiles.
Step-by-step explanation:
Alley is working with a design that includes a total of 20 tiles in various colors. To represent the portion of the design that uses red and blue tiles, she needs to write a fraction where the numerator indicates the combined number of red and blue tiles, and the denominator represents the total number of tiles used in the design. She has 5 red tiles and 4 blue tiles, so the combined number of red and blue tiles is 5 red tiles + 4 blue tiles = 9 tiles. Therefore, the numerator she should use in her fraction is 9.