Missing Addend Word Problems
Missing addend word problems are a Grade 4 skill in Saxon Math Intermediate 4 (Chapter 2). The core formula is Part + Part = Whole. When the total and one part are known, subtract to find the missing part: Missing Part = Whole - Known Part. For example: if Maria had some cookies, baked 9 more to reach 23, then she started with 23 - 9 = 14. Setting up the equation c + 9 = 23 and solving is students' first introduction to using a variable for an unknown.
Key Concepts
Property $$ \text{Some} + \text{Some more} = \text{Total} $$ $$ \text{Part} + \text{Part} = \text{Whole} $$.
Examples Jamie picked some apples, then picked 5 more to have 12 total. How many at first? $n + 5 = 12$, so $n = 12 5 = 7$ apples. A class has 25 students, 12 of whom are girls. How many are boys? $12 + b = 25$, so $b = 25 12 = 13$ boys.
Explanation When a story gives you the total but hides one of the parts, you can solve the mystery! Set up an addition equation with a variable for the missing number, then subtract the part you know from the total to find your answer.
Common Questions
What is the Part + Part = Whole formula?
The whole is the total. The two parts add up to it. When one part is unknown, use Whole - Known Part = Missing Part to solve.
Maria had cookies, baked 9 more, now has 23. How many did she start with?
Equation: c + 9 = 23. Solve: c = 23 - 9 = 14. She started with 14 cookies.
A class has 25 students, 12 are girls. How many boys?
Equation: 12 + b = 25. Solve: b = 25 - 12 = 13 boys.
How do you avoid the common error of adding instead of subtracting?
Ask: is your answer a part (smaller than the whole) or the total (bigger)? A missing part must be smaller than the whole, so subtract. Never add to find a part of a total.
A shelf has 45 books: 28 fiction, rest non-fiction. How many non-fiction?
Equation: 28 + n = 45. Solve: n = 45 - 28 = 17 non-fiction books.