Each tray has 5 colors, one color is purple, what fraction of the colors in 20 trays is purple

Each tray is 1/5 purple. So one tray is 1/5 purple, and if you add another tray, you're adding the same amount of each color, so it's now 2/10 purple, which simplifies to 1/5. No matter how many you add, the fraction will be 1/5.