1. The problem is to understand why we add 1 plus 3. 2. Addition is a basic arithmetic operation where we combine two numbers to get their total. 3. The formula for addition is $a + b = c$, where $a$ and $b$ are numbers and $c$ is their sum. 4. Here, $a = 1$ and $b = 3$. 5. Adding 1 and 3 means counting 1 unit and then adding 3 more units. 6. So, $1 + 3 = 4$. 7. This means if you have 1 object and add 3 more objects, you have 4 objects in total. 8. Addition helps us combine quantities to find a total amount.