The addition of the probabilities from certain groups of events needs to be 100%.
Since the bag is composed by green, yellow and purple marbles only the addition of the probability of green, yellow and purple marbles is 100%.
Due to the exercise gives us the probability of green marbles and yellow marbles, the difference between the 100% and the probabilities for green and yellow marbles is the probability of the purple marbles.
[tex]\begin{gathered} 100-20-50 \\ 100-70 \\ 30 \end{gathered}[/tex]The probability for purple marbles is 30%.
This probability indicates that the event could occur more times than the yellow marbles but less than the green marbles.