Learn about Applications of Addition Equations Addition Equations. So in this problem, it states that a science class is using a rain gauge to analyze weather conditions in Tampa. Supposed, the weather service report that Tampa received 3-inches of rain in December. Tampa got a total of 13-inches of rain in November and December. How many inches of rain fell in November? In this problem, we can use an addition equation to solve it. In order to do an addition equation we need to follow these steps, so our first step is that we need to write our equation. So our first step is to write our equation. When we’re writing the equation, we need to put a variable n for what we don’t know. So we are looking for the amount of rain that fell in November, so we’re going to let r=the rain in November. So our total is we know that the total is 13 inches of rain. So we know that if we add December and November together that will give us our total. So we’re going to take the rainfall in November plus the rainfall in December. And if we add these two, we should get our total rainfall for November and December. So it should be the total rainfall. So now let’s go ahead and plug in what we know. We’re looking for the rain in November so we’re going to put r for rain for November plus the rain in December is 3-inches and that will equal our total rainfall which is 13-inches. So now, we have our equation set up. So our second step is we need to isolate the unknown, so we need to isolate our variable. The variable is the unknown. So in order to isolate the unknown, you’re going to use inverse operation. So inverse operation is the opposite of the operation you’re using. So subtraction is the inverse operation of addition, so since we’re adding in this problem, the inverse operation will be a subtraction. So let me rewrite my equation, r+3=13 and now I'm going to use the inverse operation. So I'm going to subtract 3 from both sides. So I'm going to take r+3-3=13-3. Notice I'm subtracting 3 from both sides to isolate this equation. So 3-3 is 0, so I'm left with r and 13-3 is 10. So, it says that r is the rain in November, so the rain in November must be 10 inches. Now, before we move on though, we need to check our answer. So to check the answer, all we’re going to do is we’re going to substitute 10 for r. So we’re going to take our original problem, r+3=13 and I'm going to sub 10 in for r. So I'm going to put 10 where the r is and then finish my equation. So now, 10+3 is 13 equals 13. Now, since both sides of the equations are the same, this answer is correct. So, 10 inches is the correct answer for how many inches fell in November. So remember that in an equation, a quantity on the left side of the equal sign is balanced with the quantity on the right side. To solve an addition equation, we isolate the unknown using inverse operation. So, when we use inverse operation to isolate the unknown, you do the opposite of the operation. So for instance subtraction is the inverse of addition.