Chapter 2 - Set Theory - 2.1 Basic Set Concepts - Exercise Set 2.1 - Page 60: 116

{20, 21, 22}

{x| x is a time of the day when 3$\lt$ average level of happiness $\leq$ 4} In the graph we can see that at 20 hour,21 hour and 22 hour the average level of happiness was greater than 3 but less than equal to 4. So set A = {20, 21, 22}