The chance of it being less than 5 is 1/6 or 16.67%
If you start by creating a table of the possible rolls, you'll see that there are 36 options. Of those options, there are only 6 that are less than 5. The values are below.
1-1
1-2
1-3
2-1
2-2
3-1
So then you divide the number of options that fit the criteria by the overall options to get your answer.
