Balance Subtraction Equations

Goal

You need to choose a number that makes two subtraction expressions have the same value.

Key idea

Subtraction compares a starting number and a number being taken away. To balance an equation, both sides must evaluate to the same result. The blank may be in the number being subtracted or in the starting number, so pay attention to where it appears.

Method

1. Evaluate one side if it is complete. Find its value first.
2. Set the other side equal to that value.
3. Solve for the missing number.
   • If the blank is the number being subtracted, ask: “What must be taken away to get this result?”
   • If the blank is the starting number, ask: “What number minus the given amount gives this result?”
4. Check by substitution. Put your answer back into both sides and make sure they match.

Quick example idea

If one side equals 5, then the other side must also equal 5. If the blank is in a subtraction like □ − 2, choose a number that makes the result 5.

Check

After filling the blank, compute both subtraction expressions. If both values are equal, the equation is balanced.