Answer:
3 is for green marbles
21 is the total of all marbles
Explanation:
you only have 1 event so I think we don't need some formula or something
the formula is this if the event A and B has no intersection P( A U B)= P(A)+P(B)
and if it has intersection, the formula is like this P( A ∩ B)=P(A)+P(B)-P( A ∩ B)