Adding whole tens (3 addends, missing) (212)

Teaching Notes

This challenges students to work backwards. Strategy: Add the two known numbers first, then subtract from the total. Example: "I know 30 + 40 = 70, and I need 100, so the missing number is 100 - 70 = 30." Encourage students to check their answer by adding all three numbers.

Vocabulary

Addend: A number used in addition.

Missing addend: An unknown number in an addition problem.

Common Mistakes

• Subtracting in wrong order
• Adding the two known numbers and stopping
• Forgetting to check the answer
• Confusing which number is missing
• Forgetting to add the two known addends together before subtracting from the sum.
• Errors in basic addition or subtraction of whole tens.
• Misunderstanding the concept of a missing addend.

Differentiation

Discussion Questions

• What strategies can you use to add whole tens quickly?
• How does knowing about subtraction help you find the missing number in an addition problem?
• Can you explain your strategy for finding the missing whole ten to a partner?

Extension Activities

• Use a number line to find differences.
• Create equations with missing addends.
• Play 'find the missing piece'.

Parent Tip

Give your child a total and two numbers; ask them to find the missing number.

Skill Cluster

Number Sense & Algebraic Thinking

Estimated Time

15 minutes

Skills Practiced

Prerequisites

• Adding whole tens (2 addends)
• Finding missing addends (within 20)
• Understanding place value for tens and ones

Next Steps

• Adding 3-digit numbers (multiples of 100)
• Solving multi-step word problems with unknowns
• Adding whole tens with more than 3 addends