Each tray has 5 colours and one of the colour on the tray is purple. Hence 1/5 of the colours on the tray is purple and thus 1/5 of the colours on the 20 trays is purple. The fraction of colours on the trays will not change. If written out logically, 20 purples out of 100 colours in total on the trays will also given the fraction 1/5.
