4. Suppose we roll two fair dice. What's the probability that the sum of the upfaces is 10?
Here's the plot of the sample space from the section More on Probability.
First determine how many points which have coordinates that add to 10.
6  * * * * * *
Die 2 

5  * * * * * *
+


4  * * * * * *

+
3  * * * * * *


2  * * * * * *
+

1  * * * * * *
++++++Die 1
1 2 3 4 5 6