1. Let's start by understanding what a sum is. A sum is the result of adding two or more numbers together. 2. For example, if you have the numbers 3 and 5, their sum is calculated as: $$3 + 5 = 8$$ 3. Another example: if you want to add 7, 2, and 4, you do it step by step: $$7 + 2 = 9$$ Then add 4: $$9 + 4 = 13$$ So, the sum of 7, 2, and 4 is 13. 4. Important rule: When adding numbers, you can add them in any order, and the sum will be the same. This is called the commutative property of addition. 5. Also, you can group numbers in any way when adding, which is the associative property. For example: $$(3 + 5) + 2 = 3 + (5 + 2)$$ Both equal 10. 6. Practice problem: Find the sum of 10, 15, and 20. Step 1: Add 10 and 15: $$10 + 15 = 25$$ Step 2: Add 20: $$25 + 20 = 45$$ Final answer: The sum is 45.