Assessing risk in engineering, work and life

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# Example 5 Combining two independent probabilities

When tossing a coin twice, what is the probability of getting heads both times?

Solution

Using the formula, the probability of each toss scoring a head is 1 in 2, 0.5 or , and the probability of scoring two heads is

0.5 × 0.5 = 0.25,

or

This can be checked by working from first principles. Tossing two coins (or one coin twice) gives the following four possible outcomes:

• tails, tails

From this, it can be seen that the probability of getting two heads is 1 in 4, , 0.25 or 25%.

## Activity 6 Combining probability

Timing: Allow approximately 20 minutes.

Calculate the probability of the following:

• a.scoring two 6s by rolling two dice
• b.scoring six 6s by rolling six dice.

Express your answers as both a fraction and a decimal (to 2 s.f.).

• a.The probability of scoring a 6 with one roll of a die is . For two dice, applying the formula gives an overall probability of

• b.Similarly, for scoring six 6s, the overall probability is

Note that if you were to carry out these calculations using the rounded decimal value of 0.17 for the probability of a 6 (as calculated in Activity 5), you would have got the values 0.029 and 0.000 024, respectively. As this demonstrates, using rounded figures in calculations can have a significant effect.

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