Work with the sample space S of all pairs of numbers in the range 1-6, so

n(S) = 36.

(For a reminder of where this came from, go back to Unit 6.1 by pressing the "prev" button on the sidebar twice for some review on this point.)

We now take E to be the event that the sum of these numbers is 5. Thus,

E = {(1, 4), (2, 3), (3, 2), (4, 1)};
n(E) = 4.

Therefore,

P(E) =

n(E) n(S)

=

4 36

=

1 9

.