P(red) = 1/5 [that says "probability of getting red is one fifth"
P(green) = 1/5
P(blue) = 1/5
P(yellow) = 1/5
P(purple) = 1/5
The reason the fractions are all the same is that there are equal numbers of each color. For example, if there were 7 marbles of each color, there would be a total of 35 marbles.
P(red) = 7/35 = 1/5
Similar for the other colors.