When you have two or more events happening in a question:
- Break the question down into a sequence of separate events
- Find the probability of each happening
- Apply the AND/OR rule
P(Both Events Happen) = AND Rule
P(A and B) = P(A) x P(B)
So if you need to find the probability of Event A and Event B happening you multiply their separate probabilities together.
Example: Bag one contains 6 balls, 3 of which are red. Bag two contains 10 balls, 4 or which are red. What is the probability a red ball is picked from both bags?
P(red from both bags) = 3/6 x 4/10 = 0.2
P(At Least One Even Happens) = OR Rule
P(A or B) = P(A) + P(B)
So if you need to find the probability of Event A or Event B happening you add their separate probabilities together.
Example: A spinner has a green, red and blue side. The probability of it landing on each side is:
Green – 0.4
Red – 0.5
Blue – 0.1
What is the probability of it landing on green or blue?
P(lands green or blue) = 0.4 + 0.1 = 0.5