1/15
00:00

Learn more about this topic →

L.2 Subtraction up to 5: fill in the missing number

Loading questions...

Go to the practice →

"Fill in the Missing Number" in Subtraction Up to 5

In some subtraction problems, one of the numbers is missing. Students use their understanding of subtraction to figure out the missing number by thinking about what is taken away, what is left, and what was there to begin with.

Example Problems:
  • [] − 2 = 3 → Think: what number minus 2 equals 3? Answer: 5
  • 4 − [] = 2 → Think: what number must be taken away from 4 to get 2? Answer: 2
  • 5 − 1 = [] → Start at 5 and count back 1. Answer: 4
Teacher Tip

Encourage students to say subtraction problems out loud using “minus” and “equals” to strengthen their math vocabulary while solving.

Using Objects to Find the Missing Number

Physical objects, like toy fruits or counters, help students test different possibilities until they find the missing number. This hands-on method makes the process concrete.

Hands-On Examples:
  • [] − 3 = 1 → Try 4 objects: 4 − 3 = 1, so the missing number is 4.
  • 3 − [] = 0 → Removing 3 objects leaves none. The missing number is 3.
  • 5 − 0 = [] → Removing nothing leaves all 5. The missing number is 5.
Helpful Strategy

Let children try different starting amounts until they find the one that makes the subtraction sentence true.

Counting Back to Solve for the Missing Number

When the first number is known, counting back on a number line or with fingers helps find the answer. When the first number is missing, students can count forward from the result to see how many steps it takes to reach the first number.

Counting Back and Forward:
  • 4 − [] = 1 → Count forward from 1 to 4: 2, 3, 4. This is 3 steps, so the missing number is 3.
  • [] − 2 = 2 → Start at 2 and count forward 2 steps to get 4. Missing number is 4.
  • 5 − 4 = [] → Count back from 5: 5 → 4 → 3 → 2 → 1. Answer is 1.
Instructional Tip

Model both forward and backward counting to help students solve missing number problems from different starting points.

Writing Subtraction Sentences with a Missing Number

Writing subtraction sentences helps children represent a problem clearly and check their work. They learn to place the missing number in different positions and still find the answer.

Subtraction Sentences:
  • [] − 1 = 4 → Missing number is 5.
  • 3 − [] = 1 → Missing number is 2.
  • 2 − 2 = [] → Missing number is 0.
Math Language

Encourage children to read the subtraction sentence before and after they solve it to confirm it makes sense.

Checking the Answer

Students should check their work by substituting the missing number back into the subtraction sentence to see if it makes a true statement.

Checking Examples:
  • [] − 2 = 1 → Try 3: 3 − 2 = 1 ✓
  • 4 − [] = 0 → Try 4: 4 − 4 = 0 ✓
  • 5 − 1 = [] → Try 4: 5 − 1 = 4 ✓
Helpful Tip

Show students that checking is just like testing a puzzle piece to see if it fits.

Go to the practice →