A bag contains 4 red balls, 3 green balls, and 5 yellow balls. What is the probability of drawing a red or a green ball?

We have a bag contains:

4 red balls

3 green balls

5 yellow balls

Then the total number of balls in the bag is 12 balls

The chances of drawing a red ball = number of red balls / total number of balls = 4/12 = 1/3

The chances of drawing a green ball =  number of green balls/ total number of balls = 3/12 = 1/4

Now in probability the phrase OR translate as +

The the chances of drawing a red OR a green ball = chances of red + chances of green = 1/3 + 1/4 = 7/12