Answer
$$A = \{113,122,131,212,221,311\} $$
Work Step by Step
1. Write the sample space:
$$S =\{111,112,113,114,115,116,...,665, 666\}$$
The most important thing here is to notice that each sample has 3 numbers between 1 and 6.
2. Identify the ones where the sum of the three numbers is equal to 5:
$A = \{113,122,131,212,221,311\} $
