A jar contains 6 red marbles, numbered 1 to 6, and 11 blue marbles numbered
1 to 11.
Conditional probability can be done in two ways, reducing the sample space,
and by formula.
------------------------------
by reducing the sample space:
a) A marble is chosen at random. If you're told the marble is blue, what is
the probability that it has the number 3 on it?
Reduce the sample space to just the 11 blue marbles.
Only 1 of the 11 has the number 3 on it, so the probability is
1 out of 11 or 1/11.
b) The first marble is replaced, and another marble is chosen at random. If
you're told the marble has the number 1 on it, what is the probability the
marble is blue?
b) Reduce the sample space to just the 2 that are numbered 2,
One is red and the other is blue, so the probability is
1 out of 2 or 1/2.
------------------------------
by conditional probability formula
a) A marble is chosen at random. If you're told the marble is blue, what is
the probability that it has the number 3 on it?
b) The first marble is replaced, and another marble is chosen at random. If
you're told the marble has the number 1 on it, what is the probability the
marble is blue?
Edwin
