Laplace's Law

In a regular experiment, the probability of an event A, is the number of outcomes in favour for A divided by the number of outcomes of the sample space.

Example 1: In a bag we have 40 red balls, 25 green balls and 15 blue balls, all the same size. If we extract a ball, what is the probability of getting a blue ball?

Example 2: If we extract a card out of a pack of 40, what is the probability of obtaining a king?

Example 3: If we roll two dice, what is the probability that the sum of the dice is 7?

E = {(1,1), (1,2), (1,3), ….(1,6), (2,1),…(2,6), (3,1),     ….(6,6)}

  Exercises:

1.-  If we roll two dice, calculate the probability of the events:

          A= {obtain at least one 1}, B={two 6}; C={the addition is 6}

 

2.- We extract a card out of a Spanish pack of 40 cards. Calculate the probability of these events:

          A= {It is a horse}; B={it isn't a gold}; C={It is a spade or a king}

 

 

 

 Solutions: 1.- P(A)= 11/36; P(B)=1/36; P(C)=5/36; 2.- P(A)=4/40=1/10; P(B)=30/40=3/4; P(C)=13/40