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## Probability: AND & OR rule

### The ‘OR’ rule

The ‘OR’ rule states

This makes sense since the number of successes(NoS) = (NOS of A) + (NoS of B)

The number of outcomes has not changed.

So

Examples

A bag contains a red ball, 5 blue ball and 4 green balls.

1) P(red)

2) P(blue)

3) P(green)

4) P(red or green)

5) P(red or green or blue)

Answers

1)

2)

3)

4)

5)

### The ‘AND’ rule

Here we are considering the probability of something happening and then something else happening.

Let’s consider a die rolled twice.

What is the P(two sixes)?

Well we will get a ‘6’ of the time. Of these times we will get another ‘6’ of the time.

So

So

Examples

A bag contains 4 red(R) balls and 3 black(B) balls. I take out a ball, look at it and then put it back. I then take out another ball and record its colour. What is the:

- P(R,R) means getting two reds.
- P(R then B)
- P(R,B) means getting a red and blue in any order. i.e. red then blue or blue then red
Answers

So

#### Questions

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