The students are selling school pencils. if a red or green pencil is pulled from a bag, it is free. the bag contains:

5 red
10 green
15 yellow
20 blue

what is the probability that the first buyer will get a free pencil when randomly drawing one from the bag?

Blue because there are more blue pencils than any other color in the bag.

X=color pencil     Red      Green      Yellow    Blue
prob                      5/50      10/50         15/50    20/50

50 is the total number of pencils
P(X=Red U X=Green) = P(X=Red) + P(X=Green)
U means union or "or"
Since they are disjoint, you can add the probabilities together.
=5/50+10/50
=15/50
=.3
=30%