Probability Tables

## Probability Tables

Probabilities can be calculated from tables by determining the frequency of a specific event, and the total frequency for all events.

For example, a class determines how the students travel to school:

 Walk Car Bike Bus Train TOTAL Girls 5 5 2 2 1 15 Boys 4 2 3 5 2 16 TOTAL 9 7 5 7 3 31

The probability that a student travels to school by car is the total for the cars column (7) divided by the total for all journeys to school (31). P(car) = frac(7)(31).

## Example 1

The colour of each car entering a car park is noted. What is the probability of a car chosen at random being red?

 White Red Blue Green Black Silver TOTAL 8 ? 7 5 3 5 38

There are 10 red cars. The probability of it being a red car is P(red) = frac(10)(38) = frac(5)(19)

Answer: frac(5)(19)

## Example 2

The type of transport used by students to travel to school is noted below. What is the probability that a student will take a bus or a train to school?

 Walk Car Bike Bus Train TOTAL Girls 5 5 2 2 1 15 Boys 4 2 3 5 2 16 TOTAL 9 7 5 7 3 31

All students are being considered: total frequency = 31

7 students (2 + 5) travel by bus; and 3 students (1 + 2) travel by train.

P(bus or train) = frac(10)(31)

Answer: frac(10)(31)