Number
Number
Statistics and Probability
Algebra
Geometry and Measures
Mock Exams

2.6 Probability Rules

1. What is the probability getting 3 tails in a row on a coin? How much less likely to get this result compared to throwing a single tail?
The chance of throwing a tail followed by a tail, followed by another tail must be much smaller than just throwing one tail. Hence multiplying probabilities.
P(tail) and P(tail) and P(tail) = 1 2 × 1 2 × 1 2 = 1 8 = 0.125
So you are 4 times less likely to get three successive tails than throwing a single tail ( 0.5 0.125 =4 ).
2. What is the probability of rolling two 5s in succession, on a 6 die?
P(5) and P(5) = 1 6 × 1 6 = 1 36 = 0.02 7 ˙

‘AND’ Probability

The probability rule for calculating the probability of outcome of event 1 AND event 2 is as follows: P(A and B)=P(A)×P(B) Event 1 and event 2 must be independent of one another. i.e. The outcome of event 1 does not effect the probability of event 2.

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