Probability

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# Probability - PowerPoint PPT Presentation

Probability. Probability is about using maths to predict how likely something is to happen. e.g. The chance of rolling a 6 on a dice. The chance of flipping a coin and getting heads. The chance of a card being the one you guess. The chance of winning Lotto.

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### Probability

Probability is about using maths to predict how likely something is to happen.

e.g. The chance of rolling a 6 on a dice.

The chance of flipping a coin and getting heads.

The chance of a card being the one you guess.

The chance of winning Lotto.

When you are writing down how probable something is you use the letter P and a pair of brackets.

P ( )

Probability of rain P (rain)

Probability of snow P (snow)

Probability of winning P (win)

Probability of money P (money)

Fractions

The easiest way to write a probability is as a __________.

e.g.

The number of results you want.

The total number of results.

fraction

e.g. If you roll a dice there are the numbers,

1, 2, 3, 4, 5, 6

The chance of rolling a 4 is because there is one ‘4’ out of 6 possible numbers.

What is the chance of rolling a;

a) 1 b) 1 or a 2

c) even number d) number higher than 2

A deck of cards (no jokers) has 52 normal cards. 2-10 in each suit with a jack, king, queen and an ace.

If you choose a card what is the chance of

• Drawing the 5 of hearts?
• Drawing an ace of spades?
• Drawing any ace?
• Drawing a heart?
• Drawing a picture card?
• Drawing an even number card?
• Drawing a card less than 5?
Decimals and Percentages

Probabilities can also be written as decimals or percentages.

First work out the fraction probability.

To get a decimal divide the _________ number by the _________ number.

To get a percentage divide the _________ number by the _________ number and then multiply by _________.

top

bottom

top

bottom

100

Don’t copy

You are trying to phone a friend but your cell phone is flat so you have to try and remember the number. You remember all of the numbers except the second to last one.

021 555 37 ? 1 You decide to guess.

What is your chance of guessing the right number? Give your answer as a fraction, a decimal and a percentage.

What is your chance of guessing the wrong number? Give your answer as a fraction, a decimal and a percentage.

Combining probabilities

The probability of two things both happening can be found by multiplying the two probabilities.

Probability 1 x Probability 2

e.g. the chance of rolling a 6 on a dice is . The chance of rolling 2 sixes is

x =

You have 2 dice. What is the probability of rolling:

• a 1 then a 3?
• an even number then a 6?
• an odd number then an even number?
• a number bigger than 3 then a number less than 4?
• a number more than 2 then an even number?
• two numbers that are not 1?
• a number that is not 1 or 2 then a number that is 1 or 2?
• a total of 11?

A deck of cards (no jokers) has 52 normal cards. 2-10 in each suit with a jack, king, queen and an ace. You draw a card, look at it, then put it back in the deck and shuffle. What is the chance of drawing:

• a club then a heart?
• a red card then a black card?
• an ace then a black card?
• the 4 of clubs then a king.
• 2 picture cards?
• a picture card then a 5?
• the same card twice?

The of event A or event B happening is the probability of each added together minus the chance of them both happening.

This gets confusing.

We have two options for complicated questions.

• Draw it as a probability tree.
• Do a simulation.