Add Up
Add Up is a Grade 4 subtraction strategy in Saxon Math Intermediate 4 that teaches students to find a missing number in a subtraction equation by rephrasing it as addition. When b - 5 = 7, students find b by adding the two known values: 7 + 5 = 12. The method also solves for the missing subtrahend: in 15 - ? = 10, students subtract 10 from 15 to get 5. The skill also extends to algebra-style equations such as y - 38 = 54, where adding 38 to both sides isolates y as 92. Students learn the inverse relationship between addition and subtraction as a checking tool.
Key Concepts
Property To find a missing number in subtraction, you can rephrase it as addition by adding the bottom two numbers. For example, $b 5 = 7$ is the same as asking $7 + 5 = b$.
Examples To solve $b 5 = 7$, you simply “add up” the parts you know: $7 + 5 = 12$. The missing top number is $12$. For a problem like $n 10 = 4$, just add up to find the whole: $4 + 10 = 14$. So, $n$ must be $14$.
Explanation This is the super sleuth detective method! Flip the problem around and use your addition skills to crack the code. It’s often much easier to find what you started with by adding up the pieces you can see. It feels like a magic trick, but it's pure logic!
Common Questions
What is the add up strategy for subtraction?
To find the missing top number in a subtraction problem, add the two numbers you know. For b - 5 = 7, compute 7 + 5 = 12, so b = 12.
How do you solve n - 10 = 4 using add up?
Add the two known values: 4 + 10 = 14. So n = 14.
How do you solve y - 38 = 54?
Add 38 to both sides: y = 54 + 38 = 92.
Why does adding up work for finding the missing top number?
Subtraction and addition are inverse operations. The starting number (the whole) equals the part taken away plus the part remaining. Adding the two parts recovers the whole.
What is a common mistake when using the add up method?
Subtracting the two visible numbers instead of adding them. Always remember: to find the starting total, add the part removed and the part left over.