Question

A jar contains 9 red marbles numbered 1 to 9 and 3 blue marbles numbered 1 to 3. A marble is drawn at random from the jar. Find the probability that the marble is red or even-numbered?

Answer 1

There are $9+3=12$ marbles.

There are $9$ red marbles. There is one blue even-numbered marble (numbered 2).

So there are $9+1$ favorable outcomes and there are 12 total outcomes.

Then he probability that the marble is red or even-numbered is

$$P(R\cup even)=\dfrac{9+1}{9+3}=\dfrac{5}{6}$$