Practice Laplace probability
In a finite experiment with equally likely outcomes, probability is the number of favourable outcomes divided by the total number of outcomes.
Always check the equal-likelihood assumption.
Exercises with solutions
In a Laplace experiment all outcomes are equally likely. The probability is P = (number of favourable outcomes) ÷ (number of all possible outcomes). It always lies between 0 and 1, as a percentage between 0 % and 100 %.
Exercise 1
What is the probability of rolling a 4 with a die?
Show solution and working
1/6
Only the 4 is favourable, so one outcome. There are six possible outcomes: 1 ÷ 6 = 1/6.
Exercise 2
What is the probability of rolling an even number with a die?
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3/6 = 1/2
2, 4 and 6 are favourable, so three outcomes. 3 ÷ 6 = 1/2.
Exercise 3
An urn holds 5 red and 3 blue balls. What is the probability of drawing a red ball?
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5/8
5 red balls are favourable and there are 8 balls in total. So 5 ÷ 8.
Exercise 4
What is the probability of a certain event?
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1, so 100 %
If all possible outcomes are favourable the quotient is 1. A certain event has probability 1.
Common mistake
Swapping favourable and possible outcomes and inverting the fraction. And overlooking that a Laplace experiment requires all outcomes to be equally likely.
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