1. **State the problem:** We need to find the sum of the following numbers: 21.8, 20.8, 22.5, 19.7, 19.8, 23.7, 24, 20.5, 20, 23.5, 21.6, 20.5, 20.9, 19.9, 16.1, 18.5, 17.8, 20.6, 23.4, 17.6, 21.9, 20.6, 24.1, 18.7, 23.2, 21.1, 18.4, 20.1, 18.6, 18.3, 19.3, 20.8, 18.1, 15.4, 21.6, 20.2, 17.8, 16.8, 20.3, 16.6 2. **Formula used:** To find the sum of a list of numbers, we use the addition operation: $$\text{Sum} = x_1 + x_2 + x_3 + \cdots + x_n$$ where $x_i$ are the numbers in the list. 3. **Add the numbers step-by-step:** $$21.8 + 20.8 = 42.6$$ $$42.6 + 22.5 = 65.1$$ $$65.1 + 19.7 = 84.8$$ $$84.8 + 19.8 = 104.6$$ $$104.6 + 23.7 = 128.3$$ $$128.3 + 24 = 152.3$$ $$152.3 + 20.5 = 172.8$$ $$172.8 + 20 = 192.8$$ $$192.8 + 23.5 = 216.3$$ $$216.3 + 21.6 = 237.9$$ $$237.9 + 20.5 = 258.4$$ $$258.4 + 20.9 = 279.3$$ $$279.3 + 19.9 = 299.2$$ $$299.2 + 16.1 = 315.3$$ $$315.3 + 18.5 = 333.8$$ $$333.8 + 17.8 = 351.6$$ $$351.6 + 20.6 = 372.2$$ $$372.2 + 23.4 = 395.6$$ $$395.6 + 17.6 = 413.2$$ $$413.2 + 21.9 = 435.1$$ $$435.1 + 20.6 = 455.7$$ $$455.7 + 24.1 = 479.8$$ $$479.8 + 18.7 = 498.5$$ $$498.5 + 23.2 = 521.7$$ $$521.7 + 21.1 = 542.8$$ $$542.8 + 18.4 = 561.2$$ $$561.2 + 20.1 = 581.3$$ $$581.3 + 18.6 = 599.9$$ $$599.9 + 18.3 = 618.2$$ $$618.2 + 19.3 = 637.5$$ $$637.5 + 20.8 = 658.3$$ $$658.3 + 18.1 = 676.4$$ $$676.4 + 15.4 = 691.8$$ $$691.8 + 21.6 = 713.4$$ $$713.4 + 20.2 = 733.6$$ $$733.6 + 17.8 = 751.4$$ $$751.4 + 16.8 = 768.2$$ $$768.2 + 20.3 = 788.5$$ $$788.5 + 16.6 = 805.1$$ 4. **Final answer:** The sum of all the numbers is **805.1**. This method ensures accuracy by adding numbers sequentially and keeping track of the running total.