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, 25.5, 21.6, 23.5, 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 add a list of numbers, we use the summation formula: $$\text{Sum} = x_1 + x_2 + x_3 + \cdots + x_n$$ where $x_i$ are the numbers in the list. 3. **Add the numbers step-by-step:** $$21.8 + 20.8 = 42.6$$ $$42.6 + 22.5 = 65.1$$ $$65.1 + 19.7 = 84.8$$ $$84.8 + 19.8 = 104.6$$ $$104.6 + 23.7 = 128.3$$ $$128.3 + 24 = 152.3$$ $$152.3 + 20.5 = 172.8$$ $$172.8 + 20 = 192.8$$ $$192.8 + 25.5 = 218.3$$ $$218.3 + 21.6 = 239.9$$ $$239.9 + 23.5 = 263.4$$ $$263.4 + 20.9 = 284.3$$ $$284.3 + 19.9 = 304.2$$ $$304.2 + 16.1 = 320.3$$ $$320.3 + 18.5 = 338.8$$ $$338.8 + 17.8 = 356.6$$ $$356.6 + 20.6 = 377.2$$ $$377.2 + 23.4 = 400.6$$ $$400.6 + 17.6 = 418.2$$ $$418.2 + 21.9 = 440.1$$ $$440.1 + 20.6 = 460.7$$ $$460.7 + 24.1 = 484.8$$ $$484.8 + 18.7 = 503.5$$ $$503.5 + 23.2 = 526.7$$ $$526.7 + 21.1 = 547.8$$ $$547.8 + 18.4 = 566.2$$ $$566.2 + 20.1 = 586.3$$ $$586.3 + 18.6 = 604.9$$ $$604.9 + 18.3 = 623.2$$ $$623.2 + 19.3 = 642.5$$ $$642.5 + 20.8 = 663.3$$ $$663.3 + 18.1 = 681.4$$ $$681.4 + 15.4 = 696.8$$ $$696.8 + 21.6 = 718.4$$ $$718.4 + 20.2 = 738.6$$ $$738.6 + 17.8 = 756.4$$ $$756.4 + 16.8 = 773.2$$ $$773.2 + 20.3 = 793.5$$ $$793.5 + 16.6 = 810.1$$ 4. **Final answer:** The sum of all the numbers is **$810.1$**. This method ensures accuracy by adding numbers sequentially and keeping track of the running total.