Answer
$\{20, 21\}$
Work Step by Step
{x | x is a time of the day when 3 $\lt$ average level of happiness $\lt$ 4}
From the graph, we can see that this is true only at 20 and 21, because those are the only dots that are strictly between the dotted lines for 3 and 4.
Therefore, represented with the roster method, this set looks like this:
$\{20, 21\}$
